# mc (c45b2)

Monte Carlo

The Monte Carlo commands in CHARMM have been designed to allow construction

and use of an almost arbitrary move set with only a few atom selections.

This goal is accomplished by providing a pre-defined set of move types which

can be combined to specify the allowed movements of an arbitrary CHARMM

molecule. Speed and flexibility are gained by separating the bookkeeping

associated with a move (MOVE subcommands) from the actual application of

that move to the molecule (MC).

* Syntax | Syntax of MOVE and MC commands

* Description | Description of MOVE and MC commands

* Examples | Examples of MOVE and MC commands

* SA-MC simulations | How to use the SA-MC algorithm with this module

* Data Structures | Data structures shared by the MOVE and MC commands

* Shortcomings | Known problems and limitations

* References | Some references of use

The Monte Carlo commands in CHARMM have been designed to allow construction

and use of an almost arbitrary move set with only a few atom selections.

This goal is accomplished by providing a pre-defined set of move types which

can be combined to specify the allowed movements of an arbitrary CHARMM

molecule. Speed and flexibility are gained by separating the bookkeeping

associated with a move (MOVE subcommands) from the actual application of

that move to the molecule (MC).

* Syntax | Syntax of MOVE and MC commands

* Description | Description of MOVE and MC commands

* Examples | Examples of MOVE and MC commands

* SA-MC simulations | How to use the SA-MC algorithm with this module

* Data Structures | Data structures shared by the MOVE and MC commands

* Shortcomings | Known problems and limitations

* References | Some references of use

Top

Syntax for MOVE and MC commands

[Syntax MOVE < ADD | DELEte | EDIT | READ | WRITe | LINK > ]

MOVE ADD 1{ MVTP move-type } nsele{ SELE...END } -

[ WEIGht 1.0 ] [ DMAX 1.0 ] [ TFACtor 1.0 ] -

[ FEWEr 0/1 ] [ NLIMit 1 ] [ LABEL move-label ] -

[ opt-spec ] [ mini-spec ] [ hmc-spec ]

[ samc-spec ]

where nsele, the number of SELE...END statements,

depends on move-type

move-type (nsele)::= < RTRN rig-unit ( 1 ) | ! Rigid translations

RROT rig-unit ( 1+) | ! Rigid rotations

CART ( 1 ) | ! Single atom displacements

TORS ( 2 ) | ! Simple torsion rotations

CROT [PIMC] ( 1+) | ! Concerted torsion rotations

HMC ( 1 ) | ! Hybrid Monte Carlo

VOLU rig-unit ( 1 ) > ! Volume scalings

rig-unit ::= < BYREsidue | BYALl | BYHEavy | BYATom >

opt-spec ::= [ ARMP -1.0 ] [ ARMA 0.0 ] [ ARMB 0.0 ] -

[ DOMCf -1.0 ] [ ANISotropic 0 ]

mini-spec::= [ MINI < SD (default) | CONJ > ] -

[ NSTEps 0 ] [ NPRInt 0 ] [STEP 0.2 ] -

[ TOLEner 0.0 ] [ TOLGrad 0.0 ] [TOLStep 0.0 ] -

[ INBFrq -1 ]

hmc-spec ::= [ NMDSteps 0 ] [ TIMEstep 0.0 ] -

[ MEUPdate 0 ] [ MEFActor 0.0 ] [ MEWEight 0.0 ]

samc-spec::= [ SAMC ] [ WEPSilon 0.0 ] -

[ MEPSilon 0 ] [ NEPSilon 0 ]

MOVE DELEte < MVINdex move-index | LABEL move-label > -

MOVE EDIT < MVINdex move-index | LABEL move-label > -

[ WEIGht prev ] [ DMAX prev ] [ TFACtor prev ] -

[ NLIMit prev ] [ opt-spec ] [ mini-spec ] -

[ hmc-spec ]

prev ::= previous value

MOVE WRITE [UNIT -1]

MOVE READ [UNIT -1] [APPEnd 1]

MOVE LINK < MVI1 move-index | LAB1 move-label > -

[ MVI2 move-index ] [ LAB2 move-label ] [ GCMC ]

[Syntax MC]

MC [ NSTEps 0 ] [ ISEEd prev ] [ IACCept 0 ] [ PICK 0 ] -

[ TEMPerature 300.0 ] [ PRESsure 0.0 ] [ VOLUme prev ] [ ACECut 0.01 ] -

[ INBFrq 0 ] [ IMGFrq 0 ] [ IECHeck 0 ] -

[ IUNCrd -1 ] [ NSAVc 0 ] [ IMULti -1 ] -

[ RESTart ] [ IUNRead -1 ] [ IUNWrite -1 ] [ ISVFrq 0 ] -

[ IARMfrq 0 ] [ IDOMcfrq 0 ] [ gcmc-spec ] [ wl-spec ] -

[ samc-opt ]

gcmc-spec ::= [ GCBF 0.0 ] [ MUEX 0.0 ] [ DENSity 0.0 ] [ NGCTry 0 ] -

[ GCCUt 0.0 ] [ NOTBias 1 ] [ RGRId -1.0 ] [ INSPhere ] -

[ INSX 0.0 ] [ INSY 0.0 ] [ INSZ 0.0 ] [ INSR 0.0 ] -

[ XMIN 0.0 ] [ YMIN 0.0 ] [ ZMIN 0.0 ] -

[ XMAX 0.0 ] [ YMAX 0.0 ] [ ZMAX 0.0 ]

wl-spec ::= [ IWLRead -1 ] [ IWLWrite -1 ] [ NWLFrq 0 ] [ WLINcr 0.0 ] -

[ WLTOl 0.0 ] [ WLUPdate 0.0 ]

samc-opt ::= [ SAMC ] [ UNB ] [ IUNB -1 ]

Syntax for MOVE and MC commands

[Syntax MOVE < ADD | DELEte | EDIT | READ | WRITe | LINK > ]

MOVE ADD 1{ MVTP move-type } nsele{ SELE...END } -

[ WEIGht 1.0 ] [ DMAX 1.0 ] [ TFACtor 1.0 ] -

[ FEWEr 0/1 ] [ NLIMit 1 ] [ LABEL move-label ] -

[ opt-spec ] [ mini-spec ] [ hmc-spec ]

[ samc-spec ]

where nsele, the number of SELE...END statements,

depends on move-type

move-type (nsele)::= < RTRN rig-unit ( 1 ) | ! Rigid translations

RROT rig-unit ( 1+) | ! Rigid rotations

CART ( 1 ) | ! Single atom displacements

TORS ( 2 ) | ! Simple torsion rotations

CROT [PIMC] ( 1+) | ! Concerted torsion rotations

HMC ( 1 ) | ! Hybrid Monte Carlo

VOLU rig-unit ( 1 ) > ! Volume scalings

rig-unit ::= < BYREsidue | BYALl | BYHEavy | BYATom >

opt-spec ::= [ ARMP -1.0 ] [ ARMA 0.0 ] [ ARMB 0.0 ] -

[ DOMCf -1.0 ] [ ANISotropic 0 ]

mini-spec::= [ MINI < SD (default) | CONJ > ] -

[ NSTEps 0 ] [ NPRInt 0 ] [STEP 0.2 ] -

[ TOLEner 0.0 ] [ TOLGrad 0.0 ] [TOLStep 0.0 ] -

[ INBFrq -1 ]

hmc-spec ::= [ NMDSteps 0 ] [ TIMEstep 0.0 ] -

[ MEUPdate 0 ] [ MEFActor 0.0 ] [ MEWEight 0.0 ]

samc-spec::= [ SAMC ] [ WEPSilon 0.0 ] -

[ MEPSilon 0 ] [ NEPSilon 0 ]

MOVE DELEte < MVINdex move-index | LABEL move-label > -

MOVE EDIT < MVINdex move-index | LABEL move-label > -

[ WEIGht prev ] [ DMAX prev ] [ TFACtor prev ] -

[ NLIMit prev ] [ opt-spec ] [ mini-spec ] -

[ hmc-spec ]

prev ::= previous value

MOVE WRITE [UNIT -1]

MOVE READ [UNIT -1] [APPEnd 1]

MOVE LINK < MVI1 move-index | LAB1 move-label > -

[ MVI2 move-index ] [ LAB2 move-label ] [ GCMC ]

[Syntax MC]

MC [ NSTEps 0 ] [ ISEEd prev ] [ IACCept 0 ] [ PICK 0 ] -

[ TEMPerature 300.0 ] [ PRESsure 0.0 ] [ VOLUme prev ] [ ACECut 0.01 ] -

[ INBFrq 0 ] [ IMGFrq 0 ] [ IECHeck 0 ] -

[ IUNCrd -1 ] [ NSAVc 0 ] [ IMULti -1 ] -

[ RESTart ] [ IUNRead -1 ] [ IUNWrite -1 ] [ ISVFrq 0 ] -

[ IARMfrq 0 ] [ IDOMcfrq 0 ] [ gcmc-spec ] [ wl-spec ] -

[ samc-opt ]

gcmc-spec ::= [ GCBF 0.0 ] [ MUEX 0.0 ] [ DENSity 0.0 ] [ NGCTry 0 ] -

[ GCCUt 0.0 ] [ NOTBias 1 ] [ RGRId -1.0 ] [ INSPhere ] -

[ INSX 0.0 ] [ INSY 0.0 ] [ INSZ 0.0 ] [ INSR 0.0 ] -

[ XMIN 0.0 ] [ YMIN 0.0 ] [ ZMIN 0.0 ] -

[ XMAX 0.0 ] [ YMAX 0.0 ] [ ZMAX 0.0 ]

wl-spec ::= [ IWLRead -1 ] [ IWLWrite -1 ] [ NWLFrq 0 ] [ WLINcr 0.0 ] -

[ WLTOl 0.0 ] [ WLUPdate 0.0 ]

samc-opt ::= [ SAMC ] [ UNB ] [ IUNB -1 ]

Top

MOVE

The MOVE subcommands are associated with construction of the move set.

The primary MOVE subcommand is MOVE ADD, which determines all of the

locations in a subset of atoms to which a move type can be applied. For

each location (or "move instance"), MOVE ADD also determines the rotation

axes and centers, the moving atoms, and the relevant bonded terms. Thus,

each call of MOVE ADD results in a group of move instances of the same move

type (the number of instances is stored in the substitution variable ?NMVI).

By repeatedly calling the MOVE ADD command, the user can employ several

different types of moves in conjunction, which typically yields the most

efficient and complete sampling.

The available pre-defined move types are rigid translations (RTRN), rigid

rotations (RROT), single atom displacements (RTRN), rotations of individual

torsions (TORS), concerted rotation of seven (or, in the case of a chain end,

six) torsions (CROT) to deform the system locally (Dinner, 2000; Dinner, 1999;

Go and Scheraga, 1970; Dodd et al., 1993; Leontidis et al., 1994), hybrid

Monte Carlo propagations (HMC) (Duane et al., 1987; Mehlig et al., 1992),

and volume scaling moves for constant pressure simulations (Eppenga and

Frenkel, 1984). Each of these can be applied to an arbitrary subset of atoms

using standard CHARMM SELE...END statements.

MVTP rig-unit nsele Description

---- -------- ----- -----------

RTRN BYALl 1 The entire atom selection is rigidly translated.

RTRN BYREsidue 1 The residue containing each selected atom is

rigidly translated. If more than one atom in

a residue is selected, each counts as a separate

move instance.

RTRN BYHEavy 1 Each heavy atom and its associated hydrogen atoms

are rigidly translated.

RTRN BYATom 1 Each instance is a displacement of a single atom by

a random vector distributed uniformly in an ellipsoid

(see the description of the ANISotropic keyword).

For historic reasons, the CART keyword is a synonym

for RTRN BYATom, but use of the former is discouraged

since the moves are not actually based on Cartesian

coordinates.

RROT BYALl 1-2 The entire first atom selection specifies the rigid

body of atoms to be rotated, and each of the atoms in

the second atom selection is an allowed rotation

center. The second selection need not be a subset of

the first, so rotations around atoms outside the

rigid body can occur. If no second atom selection is

given (or one is given, but no atoms are selected),

the rotations are made around the center of mass of

the first atom selection.

RROT BYREsidue 1 There is only a single atom selection, and each

selected atom is a center of rotation (around a

randomly selected axis) for its residue. If more

than one atom in a residue is selected, each counts

as a separate move instance.

RROT BYHEavy 1 The hydrogens attached to a heavy atom are rigidly

rotated around the heavy atom. If the FEWEr keyword

is set to 1 (the default), a move instance is counted

for each selected heavy atom with at least one hydrogen

atom attached (whether or not the hydrogens are selected

does not matter). If the FEWEr keyword is set to 0,

all heavy atoms are counted to permit straightforward

linking with RTRN BYHEavy move groups.

TORS 2 The two selections define the middle atoms (JK in

IJKL) of the rotatable torsions. If the FEWEr keyword

is set to 1 (default is 0), the directionality of the

selection will be ignored and each rotatable bond will

be included only once in the move set (such as to rotate

the end with fewer atoms). Otherwise, each rotatable

bond will be included either once or twice depending on

whether the atom selections match the bond in only one

direction (JK) or both (JK and KJ). Only torsions in

the PSF are enumerated.

CROT 1+ The first atom selection defines the "backbone"

along which the 7 (or in the case of a chain end, 6)

dihedrals lie. Each additional pair of selections

defines non-rotatable bonds. The first bond in a set

of 6 or 7 is the driver torsion. Non-rotatable bonds

are not allowed at the third or fifth bonds following

the driver (counting only rotatable ones). Note that

there is no checking for whether bonds selected to be

rotatable are indeed so. NLIMit is the number of

torsions in addition to the driver torsion that are

restricted by the maximum rotation (DMAX); values of

0 to 5 are possible. In general, setting NLIMit

greater than or equal to 1 is recommended since it

speeds up the root finding process and moves with large

changes to the torsions tend to be rejected anyway.

Concerted rotations of path integral "polymers" require

the PIMC keyword.

HMC 1 The selected atoms are propagated with the specified

TIMEstep for NMDSteps molecular dynamics steps. The

change in total energy is used for the acceptance

criterion. SHAKE constraints are respected. The

standard fixed atom list is ignored, but note that,

in the present implementation, selections that move

only small parts of the system will be inefficient.

The non-bond list update during dynamics is separate

from that used in MC and is controlled by a common

variable set by the DYNAmics command. To suppress

updates of the non-bond list, it is necessary to

issue a dummy dynamics statement prior to MC:

DYNAmics NSTEps 0 INBFrq 0 NSAVC 0.

If IACCept=2, the dynamics take place on a transformed

potential (Andricioaei and Straub, 1996; Andricioaei

and Straub, 1997); use of the Tsallis method with HMC

requires that the TSALLIS keyword be included in

pref.dat during compilation.

If IACCept=0, sampling can be enhanced using the

method introduced in Andricioaei et al. (2003)

by setting MEFAactor, MEUPdate, and MEWEight to

non-zero values. MEFActor is the csi multiplicative

factor above Eq. 18 in the above paper, MEUPdate is

the frequency of updating the bias vector, and

MEWEight is the weight each new dynamics step carries

in the bias vector (dt/tl, where dt is the timestep

and tl is the averaging period).

VOLUme rig-unit 1 Volume scaling moves. Changes in ln V are selected

uniformly from the allowed range, and the scaling is

around the image center. The possible rigid units

are the same as for RTRN and RROT. If a "mixed"

scaling move is desired (e.g., solvent atoms are

scaled by residue while solute atoms are scaled

individually), it is necessary to couple two or more

"pure" scaling moves (see MOVE LINK).

GCMC 1 Insertion and deletion moves for grand canonical

Monte Carlo. Move instances are identified in the

same way as RTRN BYREsidue above. In other words,

there is one instance per selected atom in a grand

canonical molecule (which must be a single residue);

selecting more than one atom in a residue will waste

memory. Moreover, translation and rotation moves of

grand canonical molecules should be linked to the

GCMC move group to avoid wasting time moving inactive

atoms (see MOVE LINK). See MC and the Examples

section below for further details on grand canonical

Monte Carlo.

In addition, MOVE ADD associates with each group of move instances a set

of parameters.

The values of the following parameters are used in all MC calls.

WEIGht The relative weight of that group of move instances in

the complete move set. The probability of picking a

group of move instances with weight w_i is w_i/(sum_j w_j)

where (sum_j w_j) is the total of all the WEIGht values.

DMAX The initial maximum displacement of each instance in a

group. Translations are in angstroms and rotations are in

degrees. In cases where anisotropic automatic optimization

is to be performed, the initial assignment is isotropic.

TFACtor A multiplicative factor to scale the TEMPerature in the

acceptance criterion. TFACtor is not used in assigning

the initial velocities in HMC moves.

LABEL An optional tag for the group of move instances.

Only the first four characters are retained. All sets of

move instances are also given an integer index which can

be used instead.

The following optional parameters are associated with automatic

optimization of the volumes from which individual move instances are chosen

(the timestep in HMC moves). The two available methods are the Acceptance

Ratio Method (ARM) and Dynamically Optimized Monte Carlo (DOMC); both are

described in detail by Bouzida et al. (1992). The latter has the advantage

that it allows optimization of anisotropic volumes.

ARMP ARM target probability of move instance acceptance.

ARMA, ARMB Parameters to avoid taking the logarithm of zero in ARM:

DMAX(new) = DMAX(old)*ln(ARMA*ARMP+ARMB)/ln(ARMA*obsP+ARMB)

where obsP is the observed probability of accepting that

move instance.

DOMCF The F factor in DOMC:

DMAX(new) = DOMCF*SQRT[(d2ave*TEMP)/Eave]

where d2ave is the observed average square of the

displacement and Eave is the observed average change in

energy (both averages are done over all moves, not just those

accepted). DOMCF is used for the anisotropic version of

this equation as well. In the event that the square

root of a negative number must be taken, the routine

branches to ARM optimization, so ARMA, ARMB, and ARMP

should be set even if one plans on using DOMC.

ANISotropic DOMC anisotropic optimization of the volume from which the

moves are chosen. If ANISotropic is 0, it is off (isotropic)

and, if ANISotropic is non-zero, it is on. At present,

only 3D translation moves (RTRN and CART) allow anisotropic

optimization.

The parameters NSTEps, NPRInt, STEP, TOLEner, TOLGrad, TOLstep, and

INBFrq are associated with minimization prior to application of the acceptance

criterion (Li and Scheraga, 1987) and have the same meanings as for

MINImization (» minimiz ). Note that the INBFrq used for minimization

(set in MOVE) is distinct from that used for Monte Carlo (set in MC) even

though the command-line keywords are the same; moreover, INBFrq and NPRInt

access common variables associated with minimization directly and thus are

not stored with the rest of the move set by MOVE WRITE. During the

minimization phase of a move, all atoms that have not been constrained with

CONS FIX are considered mobile. At present, the minimization algorithms

available are steepest descents (SD) and conjugate gradients (CONJ);

in the case of CONJ, the following parameters are fixed: NCGC = 100,

PCUT = 0.9999, and TOLIter = 100. It is important that the user keep in

mind that MC with minimization does not satisfy the detailed balance

condition (microscopic reversibility) and thus should be used only for

conformational searching, not calculating equilibrium averages. Minimization

following HMC moves is not allowed.

The optional parameters SAMC, WEPSilon, MEPSilon and NEPSilon, are used for Spatial

Averaging MC simulations (SA-MC) (Refs): see the section 'SPATIAL AVERAGING SIMULATIONS'

below for more details. For each move instance the user can decide to enable

the SA-MC algorithm by putting the SAMC keyword, and then precise the 3 parameters

WEPSilon, MEPSilon, and NEPSilon.

MOVE DELEte allows the user to delete a group of move instances. The

group to be deleted is the first that matches the four-character tag specified

by LABEL or the integer specified by MVINdex; if there is a conflict, the

LABEL is used.

MOVE EDIT allows one to change the values of the parameters associated

with a group of move instances. The matching rules are the same as those for

MOVE DELEte (as a result, the LABEL parameter itself cannot be changed with

MOVE EDIT). Any parameter not specified retains its current value. If a

positive value is specified for DMAX, all move instances will be reset to

that (isotropic) value; if a negative value (default) is specified, the

maximum displacements retain their current values. If DMAX is not specified

and the ANISotropic flag changes such that anisotropy is no longer allowed

(when it was previously), the maximum displacements are assigned as the

geometric mean of the eigenvalues of the matrix used to calculate the allowed

ellipsoid from the unit sphere.

MOVE WRITe writes out the current move set to a formatted file opened

with OPEN WRITe CARD.

MOVE READ reads in a move set. If APPEnd is 0, existing moves

are eliminated; otherwise they are preserved and the new moves are appended.

MOVE ADD calls can follow to expand the move set further.

MOVE LINK links two existing moves such that they are always performed

together before applying the acceptance criterion. For example, one might

wish to perform both a rigid body translation and a rigid body rotation of

a butane molecule in the same MC step. The first move group [specified by

either its label (LAB1) or index (MVI1)] remains "active", while the second

move group becomes "slaved" to the first. In other words, a move from the

second group can no longer be selected by itself (as a result, only the WEIGht

parameter of the first move group matters). At present, move instances within

the groups are matched by indices, so the two move groups must have the same

numbers of instances. MOVE LINK can be called repeatedly to create a chain

of moves. In the example mentioned above, one might also want to change the

central dihedral of the butane molecule in the same MC step. In the second

MOVE LINK call, the second move group from the first call would become the

first move group, and the new move group would be the second:

MOVE LINK LAB1 RTRN LAB2 RROT !Resulting chain is RTRN->RROT

MOVE LINK LAB1 RROT LAB2 DIHE !Resulting chain is RTRN->RROT->DIHE

Moves can be decoupled (in the reverse order by which they were linked) by

specifying only a single move label (LAB1) or index (IND1):

MOVE LINK LAB1 RROT !Resulting chain is RTRN->RROT

MOVE LINK LAB1 RTRN !All moves are treated separately

If minimization before applying the acceptance criterion is desired, it must

be associated with the first move group in the chain (RTRN in the example);

other minimization parameters will be ignored. By the same token, only the

TFACtor for the first move group will be used. Linking is not allowed with

Hybrid Monte Carlo moves (HMC).

The GCMC keyword is used to specify that a move group involves changes to

the coordinates of molecules that are inserted and deleted during grand

canonical simulations. MOVE LINK GCMC suppresses moving "ghost" molecules,

and thus saves simulation time. In contrast to other calls to MOVE LINK,

no chain of move groups is constructed, and the non-GCMC move group is still

active (not slaved).

Please note that the MOVE LINK command is presently under development.

Consequently, the syntax might change in future versions. Moreover, its

compatibility with certain other features, such as automatic optimization

of the move limits, is not guarranteed.

MC

The MC command performs the loop over the appropriate number of Monte

Carlo steps. Each step consists of (1) randomly picking a group of move

instances (weighted), (2) randomly picking an instance from that group

(unweighted), (3) calculating the energetic contribution of the moving

atoms and their images, (4) applying the move, (5) calculating the energetic

contribution in the new configuration, (6) applying the acceptance criterion,

(7) if necessary updating the statistics for automatic optimization of the

move limits, and finally (8) performing any desired I/O (at present, only

trajectory writing is enabled).

NSTEps The number of loop iterations. Each pick of a single move

instance and subsequent application of the acceptance

criterion counts.

ISEEd The seed for the random number generator. If it is not

specified, it is unchanged, so that a script can be seeded

once initially and then loop over an MC command and yield

different behavior with each call.

TEMPerature The absolute temperature in degrees Kelvin.

PRESsure The pressure in atmospheres.

VOLUme The starting volume for constant pressure simulations.

It is only necessary to specify if the images are created

by an IMAGe TRANsformation rather than the CRYStal command.

INBFrq The non-bond list update frequency.

If INBFrq = 0, the list is not updated.

If INBFrq < 0, a heuristic is applied every -INBFrq steps;

the list is updated if any atom during a checking step moved

more than 0.5*(CUTNB - CTOFNB).

Note that a call to ENERgy or UPDAte must be made before

MC to initialize parameters for non-bond list generation.

IMGFrq The image list update frequency.

An image update will force a non-bond list update.

If IMGFrq = 0, the list is not updated.

If IMGFrq < 0, the list is updated if a heuristic non-bond

list update is done; this option should be used only if

INBFrq is also negative.

IECHeck The total energy check frequency.

If IECHeck = 0, the energy is not checked.

If IECHeck < 0, the energy is checked if a heuristic non-bond

list update is done; this option should be used only if

INBFrq is also negative.

The difference between the MC running total and the current

total is printed in the Delta-E column of the table.

NSAVc The frequency of writing out the trajectory.

If NSAVc is 0, no coordinates are written.

IUNCrd The I/O unit for trajectory writing.

RESTart If present, this keyword indicates that the run is a restart.

IUNRead The I/O unit from which to read the restart information.

IUNWrite The I/O unit to which to write the restart information.

ISVFrq The frequency of writing the restart information.

IARMfrq The frequency of updating the move size by ARM. Note that

this counter runs separately for each move instance.

If IARMfrq is 0, the move size is not updated.

IDOMcfrq The frequency of updating the move size by DOMC. Note that

this counter runs separately for each move instance.

If IDOMcfrq is 0, the move size is not updated.

If both IARMfrq and IDOMcfrq are non-zero, IARMfrq takes

priority.

PICK Flag for method of selecting moves from the move set:

0 = Random move group and random instance (default)

1 = Try each move group and instance sequentially

2 = Random move group but sequential instances within

a group

At present, the PICK flag is considered to be an unsupported

feature and may be changed without backwards compatibility in

future versions.

IACCept The acceptance criterion to be used.

If IACCept is 0, Boltzmann (Metropolis) weighting is used.

If IACCept is 1, multicanonical (constant entropy) weighting

is used (in which case TEMPerature is ignored).

If IACCept is 2, Tsallis (generalized) weighting is used.

If IACCept is 3, Wang-Landau version of the multicanonical

algorithm is used (recommended over IACCept=1).

If IACCept is 4, Spatial Averaging (SA-MC) can be used.

see the dedicated section below for more details.

The following optional parameters are specific to particular non-canical

acceptance criteria.

EMIN The estimated minimum energy of the system in Tsallis MC.

QTSAllis The Tsallis q parameter (see Andricioaei and Straub, 1997).

IMULti The I/O unit for reading in the multicanonical weights.

The file format (subject to change) is:

CHARMM title

Emin Emax Nbin

i E_i ln[n(E_i)]

.

.

.

Nbin E_Nbin ln[n(E_Nbin)]

Note that MC closes this file, so that it must be reopened

before each MC call with multicanonical weighting.

IWLRead The I/O unit from which to read the initial guesses of the

histogram and free energy for Wang-Landau MC. The file must

begin with a CHARMM title (lines starting with "*") followed

by the data in three columns: (1) the indices of the arrays

holding the accumulated histogram and free energy surface,

(2) minus the free energy divided by kT (-bF), and (3) the

accumulated histogram (n). In other words,

CHARMM title

i -bF_i n_i

.

.

.

Nbin -bF_Nbin n_Nbin

IWLWrite The I/O unit to which to write the histogram and free energy

for Wang-Landau MC. The format for the result file is similar

to that for the initial guess, except that it does not have a

title section.

NWLFrq The frequency of checking the flatness of the histogram for

Wang-Landau MC. A value on the order of 100000 is recommended.

WLINcrement The initial value of the amount added to the free energy at

each step of a Wang-Landau MC simulation. It will be halved

automatically when the criterion specified by WLUPdate is met.

WLUPdate The criterion for checking if the accumulated histogram is

flat. CHARMM MC compares this number with the ratio of

the standard deviation of the accumulated histogram to its

average. A smaller WLUP will give more accurate results for

the free energy but will require more simulation time.

Reasonable choices are typically between 0.01 and 0.05.

WLTOlerance If WLIN reaches WLTO, a Wang-Landau simulation is considered

converged and will stop prior to reaching the specified number

of MC steps. A value on the order of 10^(-8) is recommended.

The following optional parameters are associated with grand canonical

Monte Carlo simulations. Two algorithms for facilitating insertions can

be used. The cavity bias method (Mezei, 1980) generates a set of candidate

insertion positions at each GCMC step randomly, determines the ratio P_N =

cavity sites/total sites, and picks a cavity site for insertion. The

acceptance probability is adjusted based on the average of P_N to satisfy

detailed balance. Alternatively, one can keep track of the cavities with

a grid (Mezei, 1987). The grid-based method is more memory intensive and

slower at lower density but is generally more efficient at higher density

and in confined geometries such as within the binding pocket of a protein.

See Woo et al. (2004) for a discussion of the methods.

MUEX Excess chemical potential.

DENSity Desired density.

GCBFactor B = mu/kT + ln<N>, which sets the excess chemical potential.

If DENSity is greater than zero, GCBFactor will be calculated

from MUEX and DENSity.

NGCTry Number of points to generate in cavity-biased simulations

that do not employ a grid. Simple cavity bias is used

automatically if NGCTry is greater than zero.

GCCUt Cutoff distance used for evaluating whether a point in space

corresponds to a cavity in cavity-biased simulations.

RGRId The grid spacing for grid-based cavity-biased insertion.

Grid-based insertion is used automatically if RGRId is

greater than zero

INSPhere Restrict insertion to a sphere of radius INSR centered on

INSX, INSY, INSZ. Otherwise, insertions are made in the box

spanning XMIN < X < XMAX, YMIN < Y < YMAX, and

ZMIN < Z < ZMAX.

NOTBias The number of orientations to attempt when inserting.

The parameter ACECut allows approximation of the ACE screening energy

term to accelerate MC simulations which employ the ACE/ACS solvation model.

In calculating the total screening energy, as in molecular dynamics, one

performs two summations: the first determines the Born radii (b_i) and self

energies of the atoms and the second determines the screening energy given

the Born radii. In MC, this scheme becomes inefficient. One typically moves

only a small part of the system in each step, but one must update all the

pairwise interactions (between atoms i and j) in which b_i, b_j, or both

change (even if the distance between i and j remains the same). In CHARMM,

this problem is overcome by neglecting the contribution to the change in

screening energy from atom pairs in which both S_i and S_j are less than

ACECut, where S_i = Sum_k.ne.i [E_ik^self/(tau*q_i^2)] (see equations 22, 28,

and 31 of Schaefer and Karplus, 1996). For peptides, a choice of ACECut =

0.01/Angstrom has been found to yield close to the maximum increase in speed

with errors of less than 0.001 kcal/mol/step. Note that HMC moves and moves

involving minimization employ the standard ACE energy routines and thus

calculate the ACE energy exactly.

The following optional parameters are associated with Spatial Averaging

simulations (SA-MC), see the detailed section below for more details.

SAMC Keyword enabling the use of the SA-MC algorithm.

UNB Keyword enabling storage of the unbiasing factor.

IUNB The unit of the file to which the unbiasing factor is stored.

(1 column text file previously opened by the user).

MOVE

The MOVE subcommands are associated with construction of the move set.

The primary MOVE subcommand is MOVE ADD, which determines all of the

locations in a subset of atoms to which a move type can be applied. For

each location (or "move instance"), MOVE ADD also determines the rotation

axes and centers, the moving atoms, and the relevant bonded terms. Thus,

each call of MOVE ADD results in a group of move instances of the same move

type (the number of instances is stored in the substitution variable ?NMVI).

By repeatedly calling the MOVE ADD command, the user can employ several

different types of moves in conjunction, which typically yields the most

efficient and complete sampling.

The available pre-defined move types are rigid translations (RTRN), rigid

rotations (RROT), single atom displacements (RTRN), rotations of individual

torsions (TORS), concerted rotation of seven (or, in the case of a chain end,

six) torsions (CROT) to deform the system locally (Dinner, 2000; Dinner, 1999;

Go and Scheraga, 1970; Dodd et al., 1993; Leontidis et al., 1994), hybrid

Monte Carlo propagations (HMC) (Duane et al., 1987; Mehlig et al., 1992),

and volume scaling moves for constant pressure simulations (Eppenga and

Frenkel, 1984). Each of these can be applied to an arbitrary subset of atoms

using standard CHARMM SELE...END statements.

MVTP rig-unit nsele Description

---- -------- ----- -----------

RTRN BYALl 1 The entire atom selection is rigidly translated.

RTRN BYREsidue 1 The residue containing each selected atom is

rigidly translated. If more than one atom in

a residue is selected, each counts as a separate

move instance.

RTRN BYHEavy 1 Each heavy atom and its associated hydrogen atoms

are rigidly translated.

RTRN BYATom 1 Each instance is a displacement of a single atom by

a random vector distributed uniformly in an ellipsoid

(see the description of the ANISotropic keyword).

For historic reasons, the CART keyword is a synonym

for RTRN BYATom, but use of the former is discouraged

since the moves are not actually based on Cartesian

coordinates.

RROT BYALl 1-2 The entire first atom selection specifies the rigid

body of atoms to be rotated, and each of the atoms in

the second atom selection is an allowed rotation

center. The second selection need not be a subset of

the first, so rotations around atoms outside the

rigid body can occur. If no second atom selection is

given (or one is given, but no atoms are selected),

the rotations are made around the center of mass of

the first atom selection.

RROT BYREsidue 1 There is only a single atom selection, and each

selected atom is a center of rotation (around a

randomly selected axis) for its residue. If more

than one atom in a residue is selected, each counts

as a separate move instance.

RROT BYHEavy 1 The hydrogens attached to a heavy atom are rigidly

rotated around the heavy atom. If the FEWEr keyword

is set to 1 (the default), a move instance is counted

for each selected heavy atom with at least one hydrogen

atom attached (whether or not the hydrogens are selected

does not matter). If the FEWEr keyword is set to 0,

all heavy atoms are counted to permit straightforward

linking with RTRN BYHEavy move groups.

TORS 2 The two selections define the middle atoms (JK in

IJKL) of the rotatable torsions. If the FEWEr keyword

is set to 1 (default is 0), the directionality of the

selection will be ignored and each rotatable bond will

be included only once in the move set (such as to rotate

the end with fewer atoms). Otherwise, each rotatable

bond will be included either once or twice depending on

whether the atom selections match the bond in only one

direction (JK) or both (JK and KJ). Only torsions in

the PSF are enumerated.

CROT 1+ The first atom selection defines the "backbone"

along which the 7 (or in the case of a chain end, 6)

dihedrals lie. Each additional pair of selections

defines non-rotatable bonds. The first bond in a set

of 6 or 7 is the driver torsion. Non-rotatable bonds

are not allowed at the third or fifth bonds following

the driver (counting only rotatable ones). Note that

there is no checking for whether bonds selected to be

rotatable are indeed so. NLIMit is the number of

torsions in addition to the driver torsion that are

restricted by the maximum rotation (DMAX); values of

0 to 5 are possible. In general, setting NLIMit

greater than or equal to 1 is recommended since it

speeds up the root finding process and moves with large

changes to the torsions tend to be rejected anyway.

Concerted rotations of path integral "polymers" require

the PIMC keyword.

HMC 1 The selected atoms are propagated with the specified

TIMEstep for NMDSteps molecular dynamics steps. The

change in total energy is used for the acceptance

criterion. SHAKE constraints are respected. The

standard fixed atom list is ignored, but note that,

in the present implementation, selections that move

only small parts of the system will be inefficient.

The non-bond list update during dynamics is separate

from that used in MC and is controlled by a common

variable set by the DYNAmics command. To suppress

updates of the non-bond list, it is necessary to

issue a dummy dynamics statement prior to MC:

DYNAmics NSTEps 0 INBFrq 0 NSAVC 0.

If IACCept=2, the dynamics take place on a transformed

potential (Andricioaei and Straub, 1996; Andricioaei

and Straub, 1997); use of the Tsallis method with HMC

requires that the TSALLIS keyword be included in

pref.dat during compilation.

If IACCept=0, sampling can be enhanced using the

method introduced in Andricioaei et al. (2003)

by setting MEFAactor, MEUPdate, and MEWEight to

non-zero values. MEFActor is the csi multiplicative

factor above Eq. 18 in the above paper, MEUPdate is

the frequency of updating the bias vector, and

MEWEight is the weight each new dynamics step carries

in the bias vector (dt/tl, where dt is the timestep

and tl is the averaging period).

VOLUme rig-unit 1 Volume scaling moves. Changes in ln V are selected

uniformly from the allowed range, and the scaling is

around the image center. The possible rigid units

are the same as for RTRN and RROT. If a "mixed"

scaling move is desired (e.g., solvent atoms are

scaled by residue while solute atoms are scaled

individually), it is necessary to couple two or more

"pure" scaling moves (see MOVE LINK).

GCMC 1 Insertion and deletion moves for grand canonical

Monte Carlo. Move instances are identified in the

same way as RTRN BYREsidue above. In other words,

there is one instance per selected atom in a grand

canonical molecule (which must be a single residue);

selecting more than one atom in a residue will waste

memory. Moreover, translation and rotation moves of

grand canonical molecules should be linked to the

GCMC move group to avoid wasting time moving inactive

atoms (see MOVE LINK). See MC and the Examples

section below for further details on grand canonical

Monte Carlo.

In addition, MOVE ADD associates with each group of move instances a set

of parameters.

The values of the following parameters are used in all MC calls.

WEIGht The relative weight of that group of move instances in

the complete move set. The probability of picking a

group of move instances with weight w_i is w_i/(sum_j w_j)

where (sum_j w_j) is the total of all the WEIGht values.

DMAX The initial maximum displacement of each instance in a

group. Translations are in angstroms and rotations are in

degrees. In cases where anisotropic automatic optimization

is to be performed, the initial assignment is isotropic.

TFACtor A multiplicative factor to scale the TEMPerature in the

acceptance criterion. TFACtor is not used in assigning

the initial velocities in HMC moves.

LABEL An optional tag for the group of move instances.

Only the first four characters are retained. All sets of

move instances are also given an integer index which can

be used instead.

The following optional parameters are associated with automatic

optimization of the volumes from which individual move instances are chosen

(the timestep in HMC moves). The two available methods are the Acceptance

Ratio Method (ARM) and Dynamically Optimized Monte Carlo (DOMC); both are

described in detail by Bouzida et al. (1992). The latter has the advantage

that it allows optimization of anisotropic volumes.

ARMP ARM target probability of move instance acceptance.

ARMA, ARMB Parameters to avoid taking the logarithm of zero in ARM:

DMAX(new) = DMAX(old)*ln(ARMA*ARMP+ARMB)/ln(ARMA*obsP+ARMB)

where obsP is the observed probability of accepting that

move instance.

DOMCF The F factor in DOMC:

DMAX(new) = DOMCF*SQRT[(d2ave*TEMP)/Eave]

where d2ave is the observed average square of the

displacement and Eave is the observed average change in

energy (both averages are done over all moves, not just those

accepted). DOMCF is used for the anisotropic version of

this equation as well. In the event that the square

root of a negative number must be taken, the routine

branches to ARM optimization, so ARMA, ARMB, and ARMP

should be set even if one plans on using DOMC.

ANISotropic DOMC anisotropic optimization of the volume from which the

moves are chosen. If ANISotropic is 0, it is off (isotropic)

and, if ANISotropic is non-zero, it is on. At present,

only 3D translation moves (RTRN and CART) allow anisotropic

optimization.

The parameters NSTEps, NPRInt, STEP, TOLEner, TOLGrad, TOLstep, and

INBFrq are associated with minimization prior to application of the acceptance

criterion (Li and Scheraga, 1987) and have the same meanings as for

MINImization (» minimiz ). Note that the INBFrq used for minimization

(set in MOVE) is distinct from that used for Monte Carlo (set in MC) even

though the command-line keywords are the same; moreover, INBFrq and NPRInt

access common variables associated with minimization directly and thus are

not stored with the rest of the move set by MOVE WRITE. During the

minimization phase of a move, all atoms that have not been constrained with

CONS FIX are considered mobile. At present, the minimization algorithms

available are steepest descents (SD) and conjugate gradients (CONJ);

in the case of CONJ, the following parameters are fixed: NCGC = 100,

PCUT = 0.9999, and TOLIter = 100. It is important that the user keep in

mind that MC with minimization does not satisfy the detailed balance

condition (microscopic reversibility) and thus should be used only for

conformational searching, not calculating equilibrium averages. Minimization

following HMC moves is not allowed.

The optional parameters SAMC, WEPSilon, MEPSilon and NEPSilon, are used for Spatial

Averaging MC simulations (SA-MC) (Refs): see the section 'SPATIAL AVERAGING SIMULATIONS'

below for more details. For each move instance the user can decide to enable

the SA-MC algorithm by putting the SAMC keyword, and then precise the 3 parameters

WEPSilon, MEPSilon, and NEPSilon.

MOVE DELEte allows the user to delete a group of move instances. The

group to be deleted is the first that matches the four-character tag specified

by LABEL or the integer specified by MVINdex; if there is a conflict, the

LABEL is used.

MOVE EDIT allows one to change the values of the parameters associated

with a group of move instances. The matching rules are the same as those for

MOVE DELEte (as a result, the LABEL parameter itself cannot be changed with

MOVE EDIT). Any parameter not specified retains its current value. If a

positive value is specified for DMAX, all move instances will be reset to

that (isotropic) value; if a negative value (default) is specified, the

maximum displacements retain their current values. If DMAX is not specified

and the ANISotropic flag changes such that anisotropy is no longer allowed

(when it was previously), the maximum displacements are assigned as the

geometric mean of the eigenvalues of the matrix used to calculate the allowed

ellipsoid from the unit sphere.

MOVE WRITe writes out the current move set to a formatted file opened

with OPEN WRITe CARD.

MOVE READ reads in a move set. If APPEnd is 0, existing moves

are eliminated; otherwise they are preserved and the new moves are appended.

MOVE ADD calls can follow to expand the move set further.

MOVE LINK links two existing moves such that they are always performed

together before applying the acceptance criterion. For example, one might

wish to perform both a rigid body translation and a rigid body rotation of

a butane molecule in the same MC step. The first move group [specified by

either its label (LAB1) or index (MVI1)] remains "active", while the second

move group becomes "slaved" to the first. In other words, a move from the

second group can no longer be selected by itself (as a result, only the WEIGht

parameter of the first move group matters). At present, move instances within

the groups are matched by indices, so the two move groups must have the same

numbers of instances. MOVE LINK can be called repeatedly to create a chain

of moves. In the example mentioned above, one might also want to change the

central dihedral of the butane molecule in the same MC step. In the second

MOVE LINK call, the second move group from the first call would become the

first move group, and the new move group would be the second:

MOVE LINK LAB1 RTRN LAB2 RROT !Resulting chain is RTRN->RROT

MOVE LINK LAB1 RROT LAB2 DIHE !Resulting chain is RTRN->RROT->DIHE

Moves can be decoupled (in the reverse order by which they were linked) by

specifying only a single move label (LAB1) or index (IND1):

MOVE LINK LAB1 RROT !Resulting chain is RTRN->RROT

MOVE LINK LAB1 RTRN !All moves are treated separately

If minimization before applying the acceptance criterion is desired, it must

be associated with the first move group in the chain (RTRN in the example);

other minimization parameters will be ignored. By the same token, only the

TFACtor for the first move group will be used. Linking is not allowed with

Hybrid Monte Carlo moves (HMC).

The GCMC keyword is used to specify that a move group involves changes to

the coordinates of molecules that are inserted and deleted during grand

canonical simulations. MOVE LINK GCMC suppresses moving "ghost" molecules,

and thus saves simulation time. In contrast to other calls to MOVE LINK,

no chain of move groups is constructed, and the non-GCMC move group is still

active (not slaved).

Please note that the MOVE LINK command is presently under development.

Consequently, the syntax might change in future versions. Moreover, its

compatibility with certain other features, such as automatic optimization

of the move limits, is not guarranteed.

MC

The MC command performs the loop over the appropriate number of Monte

Carlo steps. Each step consists of (1) randomly picking a group of move

instances (weighted), (2) randomly picking an instance from that group

(unweighted), (3) calculating the energetic contribution of the moving

atoms and their images, (4) applying the move, (5) calculating the energetic

contribution in the new configuration, (6) applying the acceptance criterion,

(7) if necessary updating the statistics for automatic optimization of the

move limits, and finally (8) performing any desired I/O (at present, only

trajectory writing is enabled).

NSTEps The number of loop iterations. Each pick of a single move

instance and subsequent application of the acceptance

criterion counts.

ISEEd The seed for the random number generator. If it is not

specified, it is unchanged, so that a script can be seeded

once initially and then loop over an MC command and yield

different behavior with each call.

TEMPerature The absolute temperature in degrees Kelvin.

PRESsure The pressure in atmospheres.

VOLUme The starting volume for constant pressure simulations.

It is only necessary to specify if the images are created

by an IMAGe TRANsformation rather than the CRYStal command.

INBFrq The non-bond list update frequency.

If INBFrq = 0, the list is not updated.

If INBFrq < 0, a heuristic is applied every -INBFrq steps;

the list is updated if any atom during a checking step moved

more than 0.5*(CUTNB - CTOFNB).

Note that a call to ENERgy or UPDAte must be made before

MC to initialize parameters for non-bond list generation.

IMGFrq The image list update frequency.

An image update will force a non-bond list update.

If IMGFrq = 0, the list is not updated.

If IMGFrq < 0, the list is updated if a heuristic non-bond

list update is done; this option should be used only if

INBFrq is also negative.

IECHeck The total energy check frequency.

If IECHeck = 0, the energy is not checked.

If IECHeck < 0, the energy is checked if a heuristic non-bond

list update is done; this option should be used only if

INBFrq is also negative.

The difference between the MC running total and the current

total is printed in the Delta-E column of the table.

NSAVc The frequency of writing out the trajectory.

If NSAVc is 0, no coordinates are written.

IUNCrd The I/O unit for trajectory writing.

RESTart If present, this keyword indicates that the run is a restart.

IUNRead The I/O unit from which to read the restart information.

IUNWrite The I/O unit to which to write the restart information.

ISVFrq The frequency of writing the restart information.

IARMfrq The frequency of updating the move size by ARM. Note that

this counter runs separately for each move instance.

If IARMfrq is 0, the move size is not updated.

IDOMcfrq The frequency of updating the move size by DOMC. Note that

this counter runs separately for each move instance.

If IDOMcfrq is 0, the move size is not updated.

If both IARMfrq and IDOMcfrq are non-zero, IARMfrq takes

priority.

PICK Flag for method of selecting moves from the move set:

0 = Random move group and random instance (default)

1 = Try each move group and instance sequentially

2 = Random move group but sequential instances within

a group

At present, the PICK flag is considered to be an unsupported

feature and may be changed without backwards compatibility in

future versions.

IACCept The acceptance criterion to be used.

If IACCept is 0, Boltzmann (Metropolis) weighting is used.

If IACCept is 1, multicanonical (constant entropy) weighting

is used (in which case TEMPerature is ignored).

If IACCept is 2, Tsallis (generalized) weighting is used.

If IACCept is 3, Wang-Landau version of the multicanonical

algorithm is used (recommended over IACCept=1).

If IACCept is 4, Spatial Averaging (SA-MC) can be used.

see the dedicated section below for more details.

The following optional parameters are specific to particular non-canical

acceptance criteria.

EMIN The estimated minimum energy of the system in Tsallis MC.

QTSAllis The Tsallis q parameter (see Andricioaei and Straub, 1997).

IMULti The I/O unit for reading in the multicanonical weights.

The file format (subject to change) is:

CHARMM title

Emin Emax Nbin

i E_i ln[n(E_i)]

.

.

.

Nbin E_Nbin ln[n(E_Nbin)]

Note that MC closes this file, so that it must be reopened

before each MC call with multicanonical weighting.

IWLRead The I/O unit from which to read the initial guesses of the

histogram and free energy for Wang-Landau MC. The file must

begin with a CHARMM title (lines starting with "*") followed

by the data in three columns: (1) the indices of the arrays

holding the accumulated histogram and free energy surface,

(2) minus the free energy divided by kT (-bF), and (3) the

accumulated histogram (n). In other words,

CHARMM title

i -bF_i n_i

.

.

.

Nbin -bF_Nbin n_Nbin

IWLWrite The I/O unit to which to write the histogram and free energy

for Wang-Landau MC. The format for the result file is similar

to that for the initial guess, except that it does not have a

title section.

NWLFrq The frequency of checking the flatness of the histogram for

Wang-Landau MC. A value on the order of 100000 is recommended.

WLINcrement The initial value of the amount added to the free energy at

each step of a Wang-Landau MC simulation. It will be halved

automatically when the criterion specified by WLUPdate is met.

WLUPdate The criterion for checking if the accumulated histogram is

flat. CHARMM MC compares this number with the ratio of

the standard deviation of the accumulated histogram to its

average. A smaller WLUP will give more accurate results for

the free energy but will require more simulation time.

Reasonable choices are typically between 0.01 and 0.05.

WLTOlerance If WLIN reaches WLTO, a Wang-Landau simulation is considered

converged and will stop prior to reaching the specified number

of MC steps. A value on the order of 10^(-8) is recommended.

The following optional parameters are associated with grand canonical

Monte Carlo simulations. Two algorithms for facilitating insertions can

be used. The cavity bias method (Mezei, 1980) generates a set of candidate

insertion positions at each GCMC step randomly, determines the ratio P_N =

cavity sites/total sites, and picks a cavity site for insertion. The

acceptance probability is adjusted based on the average of P_N to satisfy

detailed balance. Alternatively, one can keep track of the cavities with

a grid (Mezei, 1987). The grid-based method is more memory intensive and

slower at lower density but is generally more efficient at higher density

and in confined geometries such as within the binding pocket of a protein.

See Woo et al. (2004) for a discussion of the methods.

MUEX Excess chemical potential.

DENSity Desired density.

GCBFactor B = mu/kT + ln<N>, which sets the excess chemical potential.

If DENSity is greater than zero, GCBFactor will be calculated

from MUEX and DENSity.

NGCTry Number of points to generate in cavity-biased simulations

that do not employ a grid. Simple cavity bias is used

automatically if NGCTry is greater than zero.

GCCUt Cutoff distance used for evaluating whether a point in space

corresponds to a cavity in cavity-biased simulations.

RGRId The grid spacing for grid-based cavity-biased insertion.

Grid-based insertion is used automatically if RGRId is

greater than zero

INSPhere Restrict insertion to a sphere of radius INSR centered on

INSX, INSY, INSZ. Otherwise, insertions are made in the box

spanning XMIN < X < XMAX, YMIN < Y < YMAX, and

ZMIN < Z < ZMAX.

NOTBias The number of orientations to attempt when inserting.

The parameter ACECut allows approximation of the ACE screening energy

term to accelerate MC simulations which employ the ACE/ACS solvation model.

In calculating the total screening energy, as in molecular dynamics, one

performs two summations: the first determines the Born radii (b_i) and self

energies of the atoms and the second determines the screening energy given

the Born radii. In MC, this scheme becomes inefficient. One typically moves

only a small part of the system in each step, but one must update all the

pairwise interactions (between atoms i and j) in which b_i, b_j, or both

change (even if the distance between i and j remains the same). In CHARMM,

this problem is overcome by neglecting the contribution to the change in

screening energy from atom pairs in which both S_i and S_j are less than

ACECut, where S_i = Sum_k.ne.i [E_ik^self/(tau*q_i^2)] (see equations 22, 28,

and 31 of Schaefer and Karplus, 1996). For peptides, a choice of ACECut =

0.01/Angstrom has been found to yield close to the maximum increase in speed

with errors of less than 0.001 kcal/mol/step. Note that HMC moves and moves

involving minimization employ the standard ACE energy routines and thus

calculate the ACE energy exactly.

The following optional parameters are associated with Spatial Averaging

simulations (SA-MC), see the detailed section below for more details.

SAMC Keyword enabling the use of the SA-MC algorithm.

UNB Keyword enabling storage of the unbiasing factor.

IUNB The unit of the file to which the unbiasing factor is stored.

(1 column text file previously opened by the user).

Top

EXAMPLE OF A STANDARD MC SIMULATION

No special actions must be taken during PSF generation to run an MC

simulation. Essentially, input files set up for dynamics can be turned into

MC input files by replacing the DYNAmics call with a series of MOVE ADD calls

(or a MOVE READ call) followed by a MC call. For example, to simulate a

peptide in water, one could add to the CHARMM script:

.

.

.

! Standard PSF generation and coordinate input above

! Create the MC move set

! Allow waters to move by rigid translations and rotations.

! Allow anisotropic optimization of the volume from which the

! translation vectors are chosen.

MOVE ADD MVTP RTRN BYREsidue WEIGht 2.0 DMAX 0.10 SELE (TYPE OH2) END -

ARMP 0.2 ARMA 0.8 ARMB 0.1 DOMCF 2.0 ANISo 1

MOVE ADD MVTP RROT BYREsidue WEIGht 2.0 DMAX 30.0 SELE (TYPE OH2) END -

ARMP 0.2 ARMA 0.8 ARMB 0.1 DOMCF 2.0 ANISo 0

! Allow all torsions to move by simple rotations

MOVE ADD MVTP TORS WEIGht 0.1 DMAX 30.0 FEWEr 1 -

SELE ALL END SELE ALL END

! Allow the backbone to move by concerted rotations with non-rotatable

! peptide bonds and N-CA proline bonds. If disulfides are present, the

! cysteine phi and psi should be restricted too.

MOVE ADD MVTP CROT WEIGht 0.5 DMAX 10.0 NLIMit 1 LABEL PEPTide -

SELE ((TYPE N).OR.(TYPE CA).OR.(TYPE C)) END -

SELE (TYPE C) END SELE (TYPE N) END -

SELE (RESNAME PRO .AND. TYPE CA) END -

SELE (RESNAME PRO .AND. TYPE N) END

! Seed the generator (for this example, this could be done below)

MC ISEEd 391004

OPEN WRITE UNFOrmatted UNIT 32 NAME example.trj

! Do 20000 steps at 300 K, writing energy 100 steps.

! Update the non-bonded list every 200 and

! the maximum displacements every 5 picks of that move instance

MC IACCept 0 NSTEp 20000 TEMP 300 -

INBFrq 200 IECHeck 400 IMGFrq 400 IDOMcfrq 10 -

IUNC 32 NSAVc 100

In this example, there are four groups of move instances (one for

each MOVE ADD call).

It should be mentioned that it is also possible to use moves in MC

apart from those which can be generated by MOVE ADD since the MOVE READ

command does not do any checking as it reads in the necessary move set

information. For example, it is straightforward to make rigid rotations

around a pseudo-dihedral simply by changing the pivot and moving atom lists

of a dihedral rotation. An understanding of the following section

(Data Structures) will aid in manual move creation.

GRAND CANONICAL SIMULATIONS

Some additional actions are necessary for grand canonical Monte Carlo

simulations.

Grand canonical atoms are designated as active through the GCMCon

array, which can be manipulated with the SCALAR command. A value

of 1 indicates that an atom is active and a value of 0 indicates

that an atom is inactive. It is suggested that there be roughly twice

as many grand canonical molecules as anticipated will be active on

average to accomodate fluctuations.

Atoms that block grand canonical insertions in the cavity-based

schemes described above are also initialized through a SCALAR array,

GCBLocker. A value of 1 indicates that an atom is a blocker, and

a value value of 0 indicates that it is not. Time can be saved by

excluding hydrogens.

.

.

.

! Generate PSF allowing for extra molecules.

READ SEQUENCE TIP3 432

GENERATE MAIN SETUP NOANGLE NODIHEDRAL

! Read coordinates of starting structure (216 molecules here).

OPEN READ CARD UNIT 1 NAME tip216.crd

READ COOR CARD UNIT 1

CLOSE UNIT 1

! Copy coordinates to uninitialized atoms. This is important for

! molecular liquids to define the internal structure.

COOR DUPLicate SELEct IRES 1:216 END SELEct IRES 217:432 END

! Set the active and blocking atoms

SCALar GCMC SET 1.0 SELEct IRES 1:216 END

SCALar GCMC SET 0.0 SELEct IRES 217:432 END

SCALar GCBLocker SET 1.0 SELEct TYPE OH2 END

ENERGY

! Create the MC move set

! Rigid translations

MOVE ADD MVTP RTRN BYREsidue WEIGht 1.0 DMAX 0.25 -

SELE (TYPE OH2) END LABEl DISP

! Rigid rotations

MOVE ADD MVTP RROT BYREsidue WEIGht 1.0 DMAX 30.0 -

SELE (TYPE OH2) END LABEl ROTA

! Insertion and deletion

MOVE ADD MVTP GCMC WEIGht 1.0 SELE (TYPE OH2) END LABEl GCMC

! Link the GCMC moves to the rotations and displacements to avoid moving

! inactive molecules.

MOVE LINK GCMC LAB1 GCMC LAB2 DISP

MOVE LINK GCMC LAB1 GCMC LAB2 ROTA

! Link the translations and rotations to each other for greater efficiency

MOVE LINK LAB1 DISP LAB2 ROTA

OPEN WRITE UNFOrmatted UNIT 32 NAME gcmc.trj

! Do 1000000 steps at 298 K, writing energy 1000 steps.

! Grid-based insertions with 10 attempted orientations are used.

MC NSTEp 1000000 TEMPerature 298.0 ISEEd 302430 -

INBFrq 100 IECHeck 1000 IMGFrq 100 IUNC 32 NSAVc 1000 -

MUEX -5.8 DENS 0.03342 -

RGRId 0.25 GCCUt 2.5 NOTB 10 -

XMIN -18.856 YMIN -18.856 ZMIN -18.856 -

XMAX 18.856 YMAX 18.856 ZMAX 18.856

The trajectory saved contains all of the grand canonical molecules.

The inactive coordinates are set to the initialization flag (9999.0D0) before

being written. When using the trajectory file, read the trajectory and then

delete the inactive molecules:

DELEte ATOM SELEct .BYRES. PROP X .GT. 9998.0 END

The list of active atoms is common, and thus GCMC can be combined readily with

other CHARMM features such as dynamics. In addition, it is worth noting that

the marriage of GCMC and images is not an entirely happy one; errors arising

from insufficiently frequent image updates will be minimized by making the

region in which insertion is allowed well within the primary system.

EXAMPLE OF A STANDARD MC SIMULATION

No special actions must be taken during PSF generation to run an MC

simulation. Essentially, input files set up for dynamics can be turned into

MC input files by replacing the DYNAmics call with a series of MOVE ADD calls

(or a MOVE READ call) followed by a MC call. For example, to simulate a

peptide in water, one could add to the CHARMM script:

.

.

.

! Standard PSF generation and coordinate input above

! Create the MC move set

! Allow waters to move by rigid translations and rotations.

! Allow anisotropic optimization of the volume from which the

! translation vectors are chosen.

MOVE ADD MVTP RTRN BYREsidue WEIGht 2.0 DMAX 0.10 SELE (TYPE OH2) END -

ARMP 0.2 ARMA 0.8 ARMB 0.1 DOMCF 2.0 ANISo 1

MOVE ADD MVTP RROT BYREsidue WEIGht 2.0 DMAX 30.0 SELE (TYPE OH2) END -

ARMP 0.2 ARMA 0.8 ARMB 0.1 DOMCF 2.0 ANISo 0

! Allow all torsions to move by simple rotations

MOVE ADD MVTP TORS WEIGht 0.1 DMAX 30.0 FEWEr 1 -

SELE ALL END SELE ALL END

! Allow the backbone to move by concerted rotations with non-rotatable

! peptide bonds and N-CA proline bonds. If disulfides are present, the

! cysteine phi and psi should be restricted too.

MOVE ADD MVTP CROT WEIGht 0.5 DMAX 10.0 NLIMit 1 LABEL PEPTide -

SELE ((TYPE N).OR.(TYPE CA).OR.(TYPE C)) END -

SELE (TYPE C) END SELE (TYPE N) END -

SELE (RESNAME PRO .AND. TYPE CA) END -

SELE (RESNAME PRO .AND. TYPE N) END

! Seed the generator (for this example, this could be done below)

MC ISEEd 391004

OPEN WRITE UNFOrmatted UNIT 32 NAME example.trj

! Do 20000 steps at 300 K, writing energy 100 steps.

! Update the non-bonded list every 200 and

! the maximum displacements every 5 picks of that move instance

MC IACCept 0 NSTEp 20000 TEMP 300 -

INBFrq 200 IECHeck 400 IMGFrq 400 IDOMcfrq 10 -

IUNC 32 NSAVc 100

In this example, there are four groups of move instances (one for

each MOVE ADD call).

It should be mentioned that it is also possible to use moves in MC

apart from those which can be generated by MOVE ADD since the MOVE READ

command does not do any checking as it reads in the necessary move set

information. For example, it is straightforward to make rigid rotations

around a pseudo-dihedral simply by changing the pivot and moving atom lists

of a dihedral rotation. An understanding of the following section

(Data Structures) will aid in manual move creation.

GRAND CANONICAL SIMULATIONS

Some additional actions are necessary for grand canonical Monte Carlo

simulations.

Grand canonical atoms are designated as active through the GCMCon

array, which can be manipulated with the SCALAR command. A value

of 1 indicates that an atom is active and a value of 0 indicates

that an atom is inactive. It is suggested that there be roughly twice

as many grand canonical molecules as anticipated will be active on

average to accomodate fluctuations.

Atoms that block grand canonical insertions in the cavity-based

schemes described above are also initialized through a SCALAR array,

GCBLocker. A value of 1 indicates that an atom is a blocker, and

a value value of 0 indicates that it is not. Time can be saved by

excluding hydrogens.

.

.

.

! Generate PSF allowing for extra molecules.

READ SEQUENCE TIP3 432

GENERATE MAIN SETUP NOANGLE NODIHEDRAL

! Read coordinates of starting structure (216 molecules here).

OPEN READ CARD UNIT 1 NAME tip216.crd

READ COOR CARD UNIT 1

CLOSE UNIT 1

! Copy coordinates to uninitialized atoms. This is important for

! molecular liquids to define the internal structure.

COOR DUPLicate SELEct IRES 1:216 END SELEct IRES 217:432 END

! Set the active and blocking atoms

SCALar GCMC SET 1.0 SELEct IRES 1:216 END

SCALar GCMC SET 0.0 SELEct IRES 217:432 END

SCALar GCBLocker SET 1.0 SELEct TYPE OH2 END

ENERGY

! Create the MC move set

! Rigid translations

MOVE ADD MVTP RTRN BYREsidue WEIGht 1.0 DMAX 0.25 -

SELE (TYPE OH2) END LABEl DISP

! Rigid rotations

MOVE ADD MVTP RROT BYREsidue WEIGht 1.0 DMAX 30.0 -

SELE (TYPE OH2) END LABEl ROTA

! Insertion and deletion

MOVE ADD MVTP GCMC WEIGht 1.0 SELE (TYPE OH2) END LABEl GCMC

! Link the GCMC moves to the rotations and displacements to avoid moving

! inactive molecules.

MOVE LINK GCMC LAB1 GCMC LAB2 DISP

MOVE LINK GCMC LAB1 GCMC LAB2 ROTA

! Link the translations and rotations to each other for greater efficiency

MOVE LINK LAB1 DISP LAB2 ROTA

OPEN WRITE UNFOrmatted UNIT 32 NAME gcmc.trj

! Do 1000000 steps at 298 K, writing energy 1000 steps.

! Grid-based insertions with 10 attempted orientations are used.

MC NSTEp 1000000 TEMPerature 298.0 ISEEd 302430 -

INBFrq 100 IECHeck 1000 IMGFrq 100 IUNC 32 NSAVc 1000 -

MUEX -5.8 DENS 0.03342 -

RGRId 0.25 GCCUt 2.5 NOTB 10 -

XMIN -18.856 YMIN -18.856 ZMIN -18.856 -

XMAX 18.856 YMAX 18.856 ZMAX 18.856

The trajectory saved contains all of the grand canonical molecules.

The inactive coordinates are set to the initialization flag (9999.0D0) before

being written. When using the trajectory file, read the trajectory and then

delete the inactive molecules:

DELEte ATOM SELEct .BYRES. PROP X .GT. 9998.0 END

The list of active atoms is common, and thus GCMC can be combined readily with

other CHARMM features such as dynamics. In addition, it is worth noting that

the marriage of GCMC and images is not an entirely happy one; errors arising

from insufficiently frequent image updates will be minimized by making the

region in which insertion is allowed well within the primary system.

Top

SPATIAL AVERAGING SIMULATIONS

Spatial averaging (SA-MC) is an efficient algorithm dedicated

to the study of rare-event problems: at the heart of this method is the

realization that from the equilibrium density a related, modified probability

density candidate can be constructed through a suitable transformation. This new

density is more highly connected than the original density which increases the

probability for transitions between neighboring states which in turn speeds up

the sampling. Practically, several new configurations are generated at each step

as following :

(1) Starting from a trial configuration X_0 of the system, a Gaussian distribution

of MEPSilon sets of NEPSilon configurations with standard deviation WEPSilon,

centered around X_0 is generated.

(2) The randomly chosen MC MOVE --- such as translation or rotation --- is then

applied to all MEPSilon*NEPSilon configurations and the corresponding energies

are estimated.

(3) A modified acceptance criterion (see Refs) is calculated by using those

energies.

So SA-MC uses a biased acceptance criterion: if the user is interested in some

thermodynamic properties the results have to be unbiased. Two optional parameters

(LUNB and IUNB) are used for storing in a given text file an unbiasing ratio which

can be used in a later post processing and unbiasing step (see SA-MC reference below).

Enabling SA-MC is a two steps procedure: first, for all or some of the move

instances in the input file, the user can enable SA-MC by adding the SAMC keyword,

and then give a value for WEPSilon (Gaussian width), MEPSilon (number of sets) and

NEPSilon (number of configurations):

.

.

.

! Create the MC move set by a series of calls to MOVE ADD

MOVE ADD MVTP RTRN BYATom WEIGht 1.0 DMAX 0.15 LABEL TR -

ARMP 0.40 ARMA 0.4 ARMB 0.4 DOMCf 3.0 -

SAMC WEPS 0.5 MEPS 10 NEPS 10 -

SELE ALL END

MOVE ADD MVTP TORS WEIGht 1.0 DMAX 25.0 FEWEr 1 LABEL DIHE -

ARMP 0.20 ARMA 0.4 ARMB 0.4 DOMCf 5.0 -

SAMC WEPS 0.25 MEPS 5 NEPS 5 -

SELE ALL END SELE ALL END

.

.

.

Then on the main "MC [...]" command line the user needs to set the

IACCept parameter to 4, and to add the SAMC keyword once more time. If the UNB

keyword is also added, unbiasing data is stored in the unit file IUNB

(note that this unit has to be opened before, see the following example):

.

.

.

OPEN WRITE UNFOrmatted UNIT 2 NAME test.dcd

OPEN WRITE FORMatted UNIT 10 NAME unbiasing.dat

MC TEMPerature 300.0 NSTEps 1000 IECHeck 1 IACC 4 PICK 0 -

ISEED 9921142 222455522 11142255 6221444 -

IUNCrd 2 NSAVc 1 ACECut 0.0 -

IDOM 10 -

SAMC UNB IUNB 10

.

.

.

Compatibility of the SA-MC implementation with features of the MC module (Jan. 2014):

Moves:

RTRN YES

RROT YES

TORS YES

CROT NO

HMC NO

VOLU NO

GCMC NO

MOVE LINK feature NO

Move limits optimisations

ARM and DOM optim. YES

ANISotropic optim. NO

Ensembles :

NVT (default) YES

NPT (with PRES) NO

Wang-Landau NO

SPATIAL AVERAGING SIMULATIONS

Spatial averaging (SA-MC) is an efficient algorithm dedicated

to the study of rare-event problems: at the heart of this method is the

realization that from the equilibrium density a related, modified probability

density candidate can be constructed through a suitable transformation. This new

density is more highly connected than the original density which increases the

probability for transitions between neighboring states which in turn speeds up

the sampling. Practically, several new configurations are generated at each step

as following :

(1) Starting from a trial configuration X_0 of the system, a Gaussian distribution

of MEPSilon sets of NEPSilon configurations with standard deviation WEPSilon,

centered around X_0 is generated.

(2) The randomly chosen MC MOVE --- such as translation or rotation --- is then

applied to all MEPSilon*NEPSilon configurations and the corresponding energies

are estimated.

(3) A modified acceptance criterion (see Refs) is calculated by using those

energies.

So SA-MC uses a biased acceptance criterion: if the user is interested in some

thermodynamic properties the results have to be unbiased. Two optional parameters

(LUNB and IUNB) are used for storing in a given text file an unbiasing ratio which

can be used in a later post processing and unbiasing step (see SA-MC reference below).

Enabling SA-MC is a two steps procedure: first, for all or some of the move

instances in the input file, the user can enable SA-MC by adding the SAMC keyword,

and then give a value for WEPSilon (Gaussian width), MEPSilon (number of sets) and

NEPSilon (number of configurations):

.

.

.

! Create the MC move set by a series of calls to MOVE ADD

MOVE ADD MVTP RTRN BYATom WEIGht 1.0 DMAX 0.15 LABEL TR -

ARMP 0.40 ARMA 0.4 ARMB 0.4 DOMCf 3.0 -

SAMC WEPS 0.5 MEPS 10 NEPS 10 -

SELE ALL END

MOVE ADD MVTP TORS WEIGht 1.0 DMAX 25.0 FEWEr 1 LABEL DIHE -

ARMP 0.20 ARMA 0.4 ARMB 0.4 DOMCf 5.0 -

SAMC WEPS 0.25 MEPS 5 NEPS 5 -

SELE ALL END SELE ALL END

.

.

.

Then on the main "MC [...]" command line the user needs to set the

IACCept parameter to 4, and to add the SAMC keyword once more time. If the UNB

keyword is also added, unbiasing data is stored in the unit file IUNB

(note that this unit has to be opened before, see the following example):

.

.

.

OPEN WRITE UNFOrmatted UNIT 2 NAME test.dcd

OPEN WRITE FORMatted UNIT 10 NAME unbiasing.dat

MC TEMPerature 300.0 NSTEps 1000 IECHeck 1 IACC 4 PICK 0 -

ISEED 9921142 222455522 11142255 6221444 -

IUNCrd 2 NSAVc 1 ACECut 0.0 -

IDOM 10 -

SAMC UNB IUNB 10

.

.

.

Compatibility of the SA-MC implementation with features of the MC module (Jan. 2014):

Moves:

RTRN YES

RROT YES

TORS YES

CROT NO

HMC NO

VOLU NO

GCMC NO

MOVE LINK feature NO

Move limits optimisations

ARM and DOM optim. YES

ANISotropic optim. NO

Ensembles :

NVT (default) YES

NPT (with PRES) NO

Wang-Landau NO

Top

Data Structures

MOVE ADD establishes each of the following pointers for all move types.

Each is a pointer to a dynamically allocated array that is n-instance elements

long, where n-instance is equal to the number of move instances in that group.

In all cases, if the array does not apply to a particular move, it is not

allocated.

MDXP This array contains the information about the limits of the

move. For isotropic or one-dimensional moves, it is simply

an n-instance-long array of REAL*8 elements containing the

maximum displacement. If the displacements are to be drawn

from an anisotropic volume, the array is a list of pointers,

each of which points to an array of 9 REAL*8 elements which

make up the matrix that transforms the unit sphere into the

appropriate ellipsoid.

IBLSTP A list of n-instance pointers, each of which points to

the list of bonded terms changing under that move instance.

For each element in the four-element array QBND (bonds=1,

angles=2, dihedrals=3, impropers=4) that is true, there is

an element listing the index of the final element containing

indices of that bonded term type followed by the list of

terms themselves. This list is then followed by a similar

one for the next bonded term type with QBND(i) set to true.

For example, if bonds 3, 8, and 10 and angles 16 and 17

were changing, the QBND array would contain (T T F F) and the

list would contain (4 3 8 10 7 16 17).

Urey-Bradley terms are handled with the lists generated for

angle terms, so do not get their own entries.

IPIVTP This array keeps track of any pivot or special atoms.

If there is only one pivot atom, then it is stored in the

array. If there are multiple (e.g., 2 for a TORS move

and 14 for a CROT move), the list stores a pointer to

a list containing the pivot atoms.

IMVNGP This array contains a compact list of the moving atoms.

Each element contains a pointer to a list of the following

form. The first element in the list is 1 more than the

number of rigid groups (NG). Elements 2 to NG contain the

index of the last array element with information about that

rigid group. The atoms in a rigid group are stored as

the first and last atoms in a contiguous range of atom indices.

In addition, it is worth commenting on the CHARMM fixed atom list (IMOVE)

here. MC does NOT use the fixed atom list in selecting atoms to move; rather,

atoms are held in place by judicious construction of the move set. However,

CONS FIX can be used to save memory because MC constructs the symmetrized

non-bonded list that it uses for energy calculations from the standard (upper

triangle) non-bonded list. Care must be exercised when using this feature to

avoid errors arising from moving atoms in the fixed atom list since no checking

is done.

Data Structures

MOVE ADD establishes each of the following pointers for all move types.

Each is a pointer to a dynamically allocated array that is n-instance elements

long, where n-instance is equal to the number of move instances in that group.

In all cases, if the array does not apply to a particular move, it is not

allocated.

MDXP This array contains the information about the limits of the

move. For isotropic or one-dimensional moves, it is simply

an n-instance-long array of REAL*8 elements containing the

maximum displacement. If the displacements are to be drawn

from an anisotropic volume, the array is a list of pointers,

each of which points to an array of 9 REAL*8 elements which

make up the matrix that transforms the unit sphere into the

appropriate ellipsoid.

IBLSTP A list of n-instance pointers, each of which points to

the list of bonded terms changing under that move instance.

For each element in the four-element array QBND (bonds=1,

angles=2, dihedrals=3, impropers=4) that is true, there is

an element listing the index of the final element containing

indices of that bonded term type followed by the list of

terms themselves. This list is then followed by a similar

one for the next bonded term type with QBND(i) set to true.

For example, if bonds 3, 8, and 10 and angles 16 and 17

were changing, the QBND array would contain (T T F F) and the

list would contain (4 3 8 10 7 16 17).

Urey-Bradley terms are handled with the lists generated for

angle terms, so do not get their own entries.

IPIVTP This array keeps track of any pivot or special atoms.

If there is only one pivot atom, then it is stored in the

array. If there are multiple (e.g., 2 for a TORS move

and 14 for a CROT move), the list stores a pointer to

a list containing the pivot atoms.

IMVNGP This array contains a compact list of the moving atoms.

Each element contains a pointer to a list of the following

form. The first element in the list is 1 more than the

number of rigid groups (NG). Elements 2 to NG contain the

index of the last array element with information about that

rigid group. The atoms in a rigid group are stored as

the first and last atoms in a contiguous range of atom indices.

In addition, it is worth commenting on the CHARMM fixed atom list (IMOVE)

here. MC does NOT use the fixed atom list in selecting atoms to move; rather,

atoms are held in place by judicious construction of the move set. However,

CONS FIX can be used to save memory because MC constructs the symmetrized

non-bonded list that it uses for energy calculations from the standard (upper

triangle) non-bonded list. Care must be exercised when using this feature to

avoid errors arising from moving atoms in the fixed atom list since no checking

is done.

Top

Shortcomings

In the interest of computational efficiency, Monte Carlo calls specific

energy routines directly, rather than through the main ENERGY routine. As a

result, not all energy terms are supported. Those that are supported are

bonds, angles, Urey-Bradley, dihedrals, impropers, vdw, electrostatic,

image vdw, image electrostatic, QM/MM (MOPAC only), path integral, asp-EEF1,

asp-ACE/ACS, NOE constraints, and user (also note that the user must edit

both usersb.src and mcuser.src for the user energy to work correctly). All

non-bonded calculations can be either atom- or group-based. Terms not listed

above are not included in the present implementation.

Only atom-based non-bonded lists can be used in grand canonical simulations.

No warnings are printed for attempts to move a bonded (or patched)

residue by rigid translation and rotation.

Attempts to move cross-linked residues will break MOVE ADD if

MVTP is CROT. If there is a large drift in the bond energies when

bonds lengths and angles are fixed, it is probably because a non-rotatable

bond (for example, the N-CA bond in proline) is being rotated by CROT.

Someday, a flag might be provided to choose between automatic elimination

of such moves and CROT-type relaxation of the cross-link (correct Jacobian

weighting is necessary to meet the detailed balance condition in the latter),

but such a flag is not on the immediate agenda of the MC developer.

Shortcomings

In the interest of computational efficiency, Monte Carlo calls specific

energy routines directly, rather than through the main ENERGY routine. As a

result, not all energy terms are supported. Those that are supported are

bonds, angles, Urey-Bradley, dihedrals, impropers, vdw, electrostatic,

image vdw, image electrostatic, QM/MM (MOPAC only), path integral, asp-EEF1,

asp-ACE/ACS, NOE constraints, and user (also note that the user must edit

both usersb.src and mcuser.src for the user energy to work correctly). All

non-bonded calculations can be either atom- or group-based. Terms not listed

above are not included in the present implementation.

Only atom-based non-bonded lists can be used in grand canonical simulations.

No warnings are printed for attempts to move a bonded (or patched)

residue by rigid translation and rotation.

Attempts to move cross-linked residues will break MOVE ADD if

MVTP is CROT. If there is a large drift in the bond energies when

bonds lengths and angles are fixed, it is probably because a non-rotatable

bond (for example, the N-CA bond in proline) is being rotated by CROT.

Someday, a flag might be provided to choose between automatic elimination

of such moves and CROT-type relaxation of the cross-link (correct Jacobian

weighting is necessary to meet the detailed balance condition in the latter),

but such a flag is not on the immediate agenda of the MC developer.

Top

REFERENCES

All studies that employ the MOVE and MC commands should reference:

Hu, J., Ma, A. and Dinner, A. R. (2006) Monte Carlo simulations of

biomolecules: The MC module in CHARMM. J. Comp. Chem. 27, 203-216.

In addition, studies that employ the CROT moves should reference:

Dinner, A. R. (2000) Local deformations of polymers with nonplanar rigid

main chain internal coordinates. J. Comp. Chem., 21, 1132-1144.

Grand canonical simulation studies should reference:

Woo, H.-J., Dinner, A. R. and Roux, B. (2004) Grand canonical Monte Carlo

simulation of water in protein environments. J. Chem. Phys., in press.

Studies that employ the momentum-enhanced hybrid MC should reference:

Andricioaei, I., Dinner, A. R. and Karplus, M. (2003) Self-guided enhanced

sampling methods for thermodynamic averages. J. Chem. Phys., 118,

1074-1084.

Studies that employ Wang-Landau MC should reference:

Ma, A., Nag, A. and Dinner, A. R. (2006) Dynamic coupling between coordinates

in a model for biomolecular isomerization. J. Chem. Phys. 124, 144911.

Calvo, F. (2002) Sampling along reaction coordinates with the Wang-Landau

method. Mol. Phys. 100, 3421-3427.

Wang, F. and Landau, D. P. (2001) Efficient, multiple-range random walk

algorithm to calculate the density of states. Phys. Rev. Lett. 86, 2050-2053.

Studies that employ SA-MC should reference:

Doll, J. D., Gubernatis, J. E., Plattner, N., Meuwly, M., Dupuis, P., Wang, H. (2009)

A spatial averaging approach to rare-event sampling. J. Chem. Phys., 131

Plattner, N., Doll, J. D., Meuwly, M. (2010) Spatial averaging for small molecule

diffusion in condensed phase environments. J. Chem. Phys., 133 .

Hedin, F., Meuwly, M., Plattner, N., Doll, J. D. (2014) Spatial Averaging: Sampling

Enhancement for Exploring Configurational Space of Atomic Clusters

and Biomolecules. JCTC, DOI: 10.1021/ct500529w

The following references may also be of interest:

Andricioaei, I. and Straub, J. (1997) On Monte Carlo and molecular dynamics

methods inspired by Tsallis statistics: Methodology, optimization, and

application to atomic clusters. J. Chem. Phys. 107, 9117-9124.

Andricioaei, I. and Straub, J. (1996) Generalized simulated annealing

algorithms using Tsallis statistics: Application to conformational

optimization of a tetrapeptide. Phys. Rev. E 53, R3055-R3058.

Berg, B. A. and Neuhaus, T. (1992) Multicanonical ensemble: A new approach

to simulate first-order phase transitions. Phys. Rev. Lett. 68, 9-12.

Bouzida, D., Kumar, S. and Swendsen, R. H. (1992) Efficient Monte Carlo

methods for the computer simulation of biological molecules.

Phys. Rev. A 45, 8894-8901.

Dodd, L. R., Boone, T. D. and Theodorou, D. N. (1993) A concerted

rotation algorithm for atomistic Monte Carlo simulation of polymer

melts and glasses. Mol. Phys. 78, 961-996.

Duane, S., Kennedy, A. D., Pendleton, B. J. and Roweth, D. (1987) Hybrid

Monte Carlo. Phys. Lett. B 195, 216-222.

Eppenga, R. and Frenkel, D. (1984) Monte Carlo study of the isotropic and

nematic phases of infinitely thin hard platelets. Mol. Phys. 52,

1303-1334.

Go, N. and Scheraga, H. A. (1970) Ring closure and local conformational

deformations of chain molecules. Macromolecules 3, 178-187.

Leontidis, E., de Pablo, J. J., Laso, M. and Suter, U. W. (1994)

A critical evaluation of novel algorithms for the off-lattice Monte Carlo

simulation of condensed polymer phases. Adv. Polymer Sci. 116, 285-318.

Lee, J. (1993) New Monte Carlo algorithm: Entropic sampling.

Phys. Rev. Lett. 71, 211-214.

Li, Z. and Scheraga, H. A. (1987) Monte Carlo-minimization approach to the

multiple-minima problem in protein folding. Proc. Natl. Acad. Sci. USA

84, 6611-6615.

Mehlig, B., Heermann, D. W. and Forrest, B. M. (1992) Hybrid Monte Carlo

method for condensed-matter systems. Phys. Rev. B 45, 679-685.

Metropolis, N., Rosenbluth, A. W., Rosenbluth, M. N., Teller, A. H. and

Teller, E. (1953) Equation of state calculations by fast computing

machines. J. Chem. Phys. 21, 1087-1092.

Mezei, M. (1980) A cavity-biased (T, V, mu) Monte Carlo method for the

simulation of fluids. Mol. Phys. 40, 901-906.

Mezei, M. (1987) Grand-canonical ensemble Monte Carlo study of dense liquid

Lennard-Jones, soft spheres and water. Mol. Phys. 61, 565-582.

Okamoto, Y. and Hansmann, U. H. E. (1995) Thermodynamics of helix-coil

transitions studied by multicanonical algorithms. J. Phys. Chem. 99,

11276-11287.

Schaefer, M. and Karplus, M. (1996) A comprehensive analytical treatment of

continuum electrostatics. J. Phys. Chem. 100, 1578-1599.

Tsallis, C. (1988) Possible generalization of Bolzmann-Gibbs statistics.

J. Stat. Phys. 52, 479-487.

REFERENCES

All studies that employ the MOVE and MC commands should reference:

Hu, J., Ma, A. and Dinner, A. R. (2006) Monte Carlo simulations of

biomolecules: The MC module in CHARMM. J. Comp. Chem. 27, 203-216.

In addition, studies that employ the CROT moves should reference:

Dinner, A. R. (2000) Local deformations of polymers with nonplanar rigid

main chain internal coordinates. J. Comp. Chem., 21, 1132-1144.

Grand canonical simulation studies should reference:

Woo, H.-J., Dinner, A. R. and Roux, B. (2004) Grand canonical Monte Carlo

simulation of water in protein environments. J. Chem. Phys., in press.

Studies that employ the momentum-enhanced hybrid MC should reference:

Andricioaei, I., Dinner, A. R. and Karplus, M. (2003) Self-guided enhanced

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Studies that employ Wang-Landau MC should reference:

Ma, A., Nag, A. and Dinner, A. R. (2006) Dynamic coupling between coordinates

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Studies that employ SA-MC should reference:

Doll, J. D., Gubernatis, J. E., Plattner, N., Meuwly, M., Dupuis, P., Wang, H. (2009)

A spatial averaging approach to rare-event sampling. J. Chem. Phys., 131

Plattner, N., Doll, J. D., Meuwly, M. (2010) Spatial averaging for small molecule

diffusion in condensed phase environments. J. Chem. Phys., 133 .

Hedin, F., Meuwly, M., Plattner, N., Doll, J. D. (2014) Spatial Averaging: Sampling

Enhancement for Exploring Configurational Space of Atomic Clusters

and Biomolecules. JCTC, DOI: 10.1021/ct500529w

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