struct (c41b1)
Generation and Manipulation of the Structure (PSF)
The commands described in this node are used to construct and
manipulate the PSF, the central data structure in CHARMM (see
PSF.FCM). The PSF holds lists giving every bond, bond angle, torsion
angle, and improper torsion angle as well as information needed to
generate the hydrogen bonds and the non-bonded list. It is essential
for the calculation of the energy of the system. A separate data
structure deals with symmetric images of the atoms. Images:
» chmdoc/images .
There is an order with which commands to generate and manipulate
the PSF must be given. First, segments in the PSF must be generated one
at a time. Prior to generating any segments, one must first have read a
residue topology file, » io Read. To
generate one segment, one must first read in a sequence using the READ
command, » io Sequence. Then, the GENERATE
command must be given.
Once a segment is generated, it may be manipulated. This can
be done in a very general way using the patch command. The patch
command allows, for instance, the addition of disulfide bridges,
changing the protonation state of a titratible residue or to make a
histidine heme crosslink.
The PSF can be saved with the "WRITE PSF" command. A PSF may be
read with the "READ PSF" command. The "READ PSF" command has an "APPEnd"
option that allows the merging of individual PSF files. In addition, the
"DELETE" command allows the deletetion of atoms and all references to the
deleted atoms.
* Generate | Generating a segment
* Nbx | Nonbond exclusion lists
* Patch | Multi purpose patch command to modify the PSF
* Autogen | Autogenerate angles and/or dihedrals and activate Drude particles
* Delete | Deleting atoms from the PSF
* Rename | Renaming atoms, residues, or segments
* Join | Joining two adjacent segments to form one
The commands described in this node are used to construct and
manipulate the PSF, the central data structure in CHARMM (see
PSF.FCM). The PSF holds lists giving every bond, bond angle, torsion
angle, and improper torsion angle as well as information needed to
generate the hydrogen bonds and the non-bonded list. It is essential
for the calculation of the energy of the system. A separate data
structure deals with symmetric images of the atoms. Images:
» chmdoc/images .
There is an order with which commands to generate and manipulate
the PSF must be given. First, segments in the PSF must be generated one
at a time. Prior to generating any segments, one must first have read a
residue topology file, » io Read. To
generate one segment, one must first read in a sequence using the READ
command, » io Sequence. Then, the GENERATE
command must be given.
Once a segment is generated, it may be manipulated. This can
be done in a very general way using the patch command. The patch
command allows, for instance, the addition of disulfide bridges,
changing the protonation state of a titratible residue or to make a
histidine heme crosslink.
The PSF can be saved with the "WRITE PSF" command. A PSF may be
read with the "READ PSF" command. The "READ PSF" command has an "APPEnd"
option that allows the merging of individual PSF files. In addition, the
"DELETE" command allows the deletetion of atoms and all references to the
deleted atoms.
* Generate | Generating a segment
* Nbx | Nonbond exclusion lists
* Patch | Multi purpose patch command to modify the PSF
* Autogen | Autogenerate angles and/or dihedrals and activate Drude particles
* Delete | Deleting atoms from the PSF
* Rename | Renaming atoms, residues, or segments
* Join | Joining two adjacent segments to form one
Top
The Generate Command - Construct a Segment of the PSF
[Syntax GENErate segment]
GENErate [segid] { generate-spec } [SETUp]
{ DUPLicate segid }
generate-spec::= [FIRSt pres] [LAST pres] [WARN] [ ANGLe ] [ DIHEdrals ]
[ NOANgle ] [ NODIhedral]
Function
This command uses the sequence of residues specified in the last
READ SEQUuence command and the information stored in the residue
topology file to add the next segment to the PSF. Each segment contains a
list of all the bonds, angles, dihedral angles, and improper torsions
needed to calculate the energy. It also assigns charges to all the
atoms, sets up the nonbonded exclusions list, and specifies hydrogen
bond donors and acceptors. Any internal coordinate which references
atoms outside the range of the segment is deleted. This prevents any
unexpected bonding of segments.
The FIRSt and LAST specifications define what patch-residues
should be used for the terminating residues. If no specification is given,
then the default patching as specified in the topology file will be used.
The WARN keyword, will list all elements that were deleted due
to nonexistant atoms (usually references to the terminating residues).
The SETUp option will cause any internal coordinate table entries
(IC) from the topology file to be appended to the main IC table.
The ANGLe (NOANgle) and DIHEdral (NODIhedral) options overide
the autogeneration option specified in the topology files or on the
command line. This may be done to supress unwanted additional terms,
or to add terms for specific residues.
NOTE: The solvent residues (TIP3, ST2, WAT) must be generated
with the NOANgle and/or NODIhedral qualifier. This is only necessary for
the files which use the AUTOgenerate ANGLes and/or DIHEdrals as a
default. This also means that a protein residue sequence and
water molecules may not be combined in the same generate command.
Also, there is a special "READ SEQUence residue_type integer" command where
integer is the number of resudies of residue_type (often water molecules).
This avoids the need to list the number of residues followed by the
specification of each TIP3 residue name individually as is done with a
protein.
For the DUPLicate segment option, the generate command MUST NOT
be preceeded by a READ SEQUence command. This option will create a new
segment which is identical (except for the segid) to an existing segment.
This option is mainly intended for the use in setting up small crystals
for viewing and other analysis.
The Generate Command - Construct a Segment of the PSF
[Syntax GENErate segment]
GENErate [segid] { generate-spec } [SETUp]
{ DUPLicate segid }
generate-spec::= [FIRSt pres] [LAST pres] [WARN] [ ANGLe ] [ DIHEdrals ]
[ NOANgle ] [ NODIhedral]
Function
This command uses the sequence of residues specified in the last
READ SEQUuence command and the information stored in the residue
topology file to add the next segment to the PSF. Each segment contains a
list of all the bonds, angles, dihedral angles, and improper torsions
needed to calculate the energy. It also assigns charges to all the
atoms, sets up the nonbonded exclusions list, and specifies hydrogen
bond donors and acceptors. Any internal coordinate which references
atoms outside the range of the segment is deleted. This prevents any
unexpected bonding of segments.
The FIRSt and LAST specifications define what patch-residues
should be used for the terminating residues. If no specification is given,
then the default patching as specified in the topology file will be used.
The WARN keyword, will list all elements that were deleted due
to nonexistant atoms (usually references to the terminating residues).
The SETUp option will cause any internal coordinate table entries
(IC) from the topology file to be appended to the main IC table.
The ANGLe (NOANgle) and DIHEdral (NODIhedral) options overide
the autogeneration option specified in the topology files or on the
command line. This may be done to supress unwanted additional terms,
or to add terms for specific residues.
NOTE: The solvent residues (TIP3, ST2, WAT) must be generated
with the NOANgle and/or NODIhedral qualifier. This is only necessary for
the files which use the AUTOgenerate ANGLes and/or DIHEdrals as a
default. This also means that a protein residue sequence and
water molecules may not be combined in the same generate command.
Also, there is a special "READ SEQUence residue_type integer" command where
integer is the number of resudies of residue_type (often water molecules).
This avoids the need to list the number of residues followed by the
specification of each TIP3 residue name individually as is done with a
protein.
For the DUPLicate segment option, the generate command MUST NOT
be preceeded by a READ SEQUence command. This option will create a new
segment which is identical (except for the segid) to an existing segment.
This option is mainly intended for the use in setting up small crystals
for viewing and other analysis.
Top
Some pairs of atoms are excluded from the nbond exclusion lists
because their interactions are described by other terms in the hamiltonian.
By default directly bonded atoms and the 1-3 atoms of an angle are excluded
from the nonbond calculation. In addition the diagonal interactions of
the six membered rings in tyrosine and phenylalanine were excluded from
the nonbond calculation through charmm version 15 with RTOPH6. Hydrogen
bonds, and dihedral 1-4 interactions are not excluded (note that other
workers may differ from us on one or both of these points).
The list of nonbonded exclusion is generated in two steps. First
a preliminary list is made at generation by GENIC using any information
that may be present in the topology file (for example, diagonal
interactions in rings). The second step is an automatic compilation of
all the bond and angle interactions, followed by a sorting of the list,
performed in MAKINB. The list is stored in the linked list pair IBLO14/INB14,
where IBLO14(i) points to the last exclusion in INB14 to atom i. If the list
is modified after MAKINB, then either MAKINB should be called again to
resort the list, or care must be taken to see that the INB14 list is ascending
with all INB14 entries having higher atom numbers than i and that all atoms
have at least one INB entry.
MAKINB is called by default after any operation which changes
internal coordinates such as generate, patch, or edit.
The exclusion list can be specified in three ways. First, interactions
that are to be excluded can be placed in the topology file by listing
the excluded atoms after the charge. Second,
NBXM mode can be specified as a qualifier to any of the commands which
change internal coordinates. Third, the default NBXM value can be specified
in the parameter file. The NBXM values and actions are (in the
following "include" refers to what is being kept (included) in the
exclusion list):
0 use the existing list (do nothing)
1 or -1 include nothing extra
2 or -2 include only 1-2 (bond) interactions
3 or -3 also include 1-3 (angle) interactions
4 or -4 also include 1-4 interactions automatically.
5 or -5 include up to 1-3 interactions as exclusions and process
1-4 interactions using the 1-4 van der Waal parameters and
reduced elecrostatics (E14FAC).
Negative values suppress the use of the information present in
the topology file. Positive values add to the information that was in
the topology file.
Some pairs of atoms are excluded from the nbond exclusion lists
because their interactions are described by other terms in the hamiltonian.
By default directly bonded atoms and the 1-3 atoms of an angle are excluded
from the nonbond calculation. In addition the diagonal interactions of
the six membered rings in tyrosine and phenylalanine were excluded from
the nonbond calculation through charmm version 15 with RTOPH6. Hydrogen
bonds, and dihedral 1-4 interactions are not excluded (note that other
workers may differ from us on one or both of these points).
The list of nonbonded exclusion is generated in two steps. First
a preliminary list is made at generation by GENIC using any information
that may be present in the topology file (for example, diagonal
interactions in rings). The second step is an automatic compilation of
all the bond and angle interactions, followed by a sorting of the list,
performed in MAKINB. The list is stored in the linked list pair IBLO14/INB14,
where IBLO14(i) points to the last exclusion in INB14 to atom i. If the list
is modified after MAKINB, then either MAKINB should be called again to
resort the list, or care must be taken to see that the INB14 list is ascending
with all INB14 entries having higher atom numbers than i and that all atoms
have at least one INB entry.
MAKINB is called by default after any operation which changes
internal coordinates such as generate, patch, or edit.
The exclusion list can be specified in three ways. First, interactions
that are to be excluded can be placed in the topology file by listing
the excluded atoms after the charge. Second,
NBXM mode can be specified as a qualifier to any of the commands which
change internal coordinates. Third, the default NBXM value can be specified
in the parameter file. The NBXM values and actions are (in the
following "include" refers to what is being kept (included) in the
exclusion list):
0 use the existing list (do nothing)
1 or -1 include nothing extra
2 or -2 include only 1-2 (bond) interactions
3 or -3 also include 1-3 (angle) interactions
4 or -4 also include 1-4 interactions automatically.
5 or -5 include up to 1-3 interactions as exclusions and process
1-4 interactions using the 1-4 van der Waal parameters and
reduced elecrostatics (E14FAC).
Negative values suppress the use of the information present in
the topology file. Positive values add to the information that was in
the topology file.
Top
Patch command to modify PSF
[SYNTAX PATCh structure file]
Syntax (command level)
PATCh <pres-name> segid1 resid1 [, segid2 resid2 [,...
[, segid9 resid9]...]]
[SORT]
[SETUp]
[WARN]
Syntax (corresponding patch residue in RTF)
PRES <pres-name>
[GROUp]
[ATOM <I><atomname> <parameter type> <charge> ]
[DELEte ATOM <I><atomname>]
[ [DELEte] BOND <I1> <I2> ]
[ [DELEte] ANGLe <I1> <I2> <I3> ]
[ [DELEte] DIHEdral <I1> <I2> <I3> <I4> ]
[ [DELEte] IMPRoper <I1> <I2> <I3> <I4> ]
[ [DELEte] DONOr [<I1>] <I2> [[<I3> [<I4>]] ]
[ [DELEte] ACCEptor <I1> [ <I2> [ <I3> ]] ]
[ IC <I1> <I2> [*]<I3> <I4> real real real real real ]
[ DELEte IC <I1> <I2> [*]<I3> <I4> ]
where I1, I2, I3, I4 refer to <I><atomname>.
Rules governing the patch procedure:
1) If an atom is being added via a PATCH at least one or more atoms
already existing in the residue to which the patch is being added
must be included in the PRES with an ATOM statement. Unless
this(these) atoms are deleted using the DELEte ATOM command
internal terms associated with this atom which are already present
in the residue should NOT be included in the PRES.
2) if no <I> is specified before <atomname> the patch procedure assumes
that the atom should be in residue (segid1 resid1).
3) a '-', '+', '#' as a first letter in <atomname> tries to locate or add
the atom <atomname> in the previous, next, next of the next, residue
of residue (segid<I> resid<I>), respectively.
4) GROUP brackets in a patch residue have highest priority.
5) If no GROUP is specified, the group numbers of referenced, already
existing atoms remain unchanged. Added atoms are placed in the last group
of the referenced residue.
6) A GROUP statement in a patch residue CAN enclose atoms in different
referenced residues. However, if there is a conflict between
sequential residue AND group boundaries new residues MIGHT be created
with resid's and segid's referring to the referenced residues.
These cases are indicated by a message from MAPIC that a negative number
of residues were created. The user has to check the PSF explicitly
to decide whether the modifications done by PATCH are appropriate.
7) Along with the PSF the coordinates, comparision coordinates, harmonic
constraints, fixed atom list, internal coordinates (IC) are
mapped correctly.
8) THERE IS NO MAP OF NBONDS, HBONDS, SHAKE, DYNAMICS ETC.
THE ATOMNUMBERS ARE CHANGED.
9) Any bond, angle, etc referring to deleted atoms is itself deleted.
The bond, angle, etc lists are compressed.
10) Along with the AUTOgenerate ANGLe and/or DIHEdral options it is
also necessary to invoke PATCH in order for angles/and/or dihedrals
to be automatically generated after invoking a patch. If this is
not done angles and/or dihedrals have to be included in the PRES
when that particular patch is being called after the GENErate
statement. The angles and/or dihedrals will be generated
automatically for any patch which is called in the GENErate
statement following the FIRSt or LAST statements. NOTE: If angles
and dihedrals are present in a PRES which is called in a GENErate
statement in which AUTOgenerate ANGLes and/or DIHEdrals is being
used those angles and/or dihedrals will be invoked twice in the PSF
and, thus, be included twice when the energy is calculated. As of
c40b1 the AUTOGENERATE PATCH command is included in the topology
files there be avoiding the inclusion of angles and dihedral with
PRES or specifying the AUTOgenerate option after patches.
The AUTOgenerate command (next) can be used to circumvent the above
problems, and removes the need for specifying angles and dihedrals
as part of a PRES definition.
Patch command to modify PSF
[SYNTAX PATCh structure file]
Syntax (command level)
PATCh <pres-name> segid1 resid1 [, segid2 resid2 [,...
[, segid9 resid9]...]]
[SORT]
[SETUp]
[WARN]
Syntax (corresponding patch residue in RTF)
PRES <pres-name>
[GROUp]
[ATOM <I><atomname> <parameter type> <charge> ]
[DELEte ATOM <I><atomname>]
[ [DELEte] BOND <I1> <I2> ]
[ [DELEte] ANGLe <I1> <I2> <I3> ]
[ [DELEte] DIHEdral <I1> <I2> <I3> <I4> ]
[ [DELEte] IMPRoper <I1> <I2> <I3> <I4> ]
[ [DELEte] DONOr [<I1>] <I2> [[<I3> [<I4>]] ]
[ [DELEte] ACCEptor <I1> [ <I2> [ <I3> ]] ]
[ IC <I1> <I2> [*]<I3> <I4> real real real real real ]
[ DELEte IC <I1> <I2> [*]<I3> <I4> ]
where I1, I2, I3, I4 refer to <I><atomname>.
Rules governing the patch procedure:
1) If an atom is being added via a PATCH at least one or more atoms
already existing in the residue to which the patch is being added
must be included in the PRES with an ATOM statement. Unless
this(these) atoms are deleted using the DELEte ATOM command
internal terms associated with this atom which are already present
in the residue should NOT be included in the PRES.
2) if no <I> is specified before <atomname> the patch procedure assumes
that the atom should be in residue (segid1 resid1).
3) a '-', '+', '#' as a first letter in <atomname> tries to locate or add
the atom <atomname> in the previous, next, next of the next, residue
of residue (segid<I> resid<I>), respectively.
4) GROUP brackets in a patch residue have highest priority.
5) If no GROUP is specified, the group numbers of referenced, already
existing atoms remain unchanged. Added atoms are placed in the last group
of the referenced residue.
6) A GROUP statement in a patch residue CAN enclose atoms in different
referenced residues. However, if there is a conflict between
sequential residue AND group boundaries new residues MIGHT be created
with resid's and segid's referring to the referenced residues.
These cases are indicated by a message from MAPIC that a negative number
of residues were created. The user has to check the PSF explicitly
to decide whether the modifications done by PATCH are appropriate.
7) Along with the PSF the coordinates, comparision coordinates, harmonic
constraints, fixed atom list, internal coordinates (IC) are
mapped correctly.
8) THERE IS NO MAP OF NBONDS, HBONDS, SHAKE, DYNAMICS ETC.
THE ATOMNUMBERS ARE CHANGED.
9) Any bond, angle, etc referring to deleted atoms is itself deleted.
The bond, angle, etc lists are compressed.
10) Along with the AUTOgenerate ANGLe and/or DIHEdral options it is
also necessary to invoke PATCH in order for angles/and/or dihedrals
to be automatically generated after invoking a patch. If this is
not done angles and/or dihedrals have to be included in the PRES
when that particular patch is being called after the GENErate
statement. The angles and/or dihedrals will be generated
automatically for any patch which is called in the GENErate
statement following the FIRSt or LAST statements. NOTE: If angles
and dihedrals are present in a PRES which is called in a GENErate
statement in which AUTOgenerate ANGLes and/or DIHEdrals is being
used those angles and/or dihedrals will be invoked twice in the PSF
and, thus, be included twice when the energy is calculated. As of
c40b1 the AUTOGENERATE PATCH command is included in the topology
files there be avoiding the inclusion of angles and dihedral with
PRES or specifying the AUTOgenerate option after patches.
The AUTOgenerate command (next) can be used to circumvent the above
problems, and removes the need for specifying angles and dihedrals
as part of a PRES definition.
Top
Completely autogenerate all angles and/or dihedrals
AUTOgen { ANGLes [ DIHEdrals ] PATCh }
{ DIHEdrals [ ANGLes ] PATCh }
{ OFF }
{ DRUDe }
ANGLes and/or DIHEdrals activates the setting of the angle and/or
dihedral counts to zero in the PSF following a GENErate command, and
rebuilds the indicated list(s) of energy terms. PATCh activates
autogeneration for patches, otherwise AUTOgenerate needs to be
specified after patching. In general autogenerate simplifies the
development of residues and patches, since only bonding terms need to
be specified in RESI and PRES definitions. Note that at least one
keyword is required. DRUDe turns on the creation of drude particles
when reading an RTF. This only occurs with atoms that include the
ALPHA keyword after the charge.
WARNING: may be a problem if the PSF contains any water molecules. In
that case generate water last and include NOANgle NODIhedral in the
GENErate command.
Completely autogenerate all angles and/or dihedrals
AUTOgen { ANGLes [ DIHEdrals ] PATCh }
{ DIHEdrals [ ANGLes ] PATCh }
{ OFF }
{ DRUDe }
ANGLes and/or DIHEdrals activates the setting of the angle and/or
dihedral counts to zero in the PSF following a GENErate command, and
rebuilds the indicated list(s) of energy terms. PATCh activates
autogeneration for patches, otherwise AUTOgenerate needs to be
specified after patching. In general autogenerate simplifies the
development of residues and patches, since only bonding terms need to
be specified in RESI and PRES definitions. Note that at least one
keyword is required. DRUDe turns on the creation of drude particles
when reading an RTF. This only occurs with atoms that include the
ALPHA keyword after the charge.
WARNING: may be a problem if the PSF contains any water molecules. In
that case generate water last and include NOANgle NODIhedral in the
GENErate command.
Top
Delete atoms or energy terms in the structure
[Syntax DELEte terms in structure file]
DELEte { ATOMs atom-selection } [SORT]
{ }
{ { BONDs } double-atom-selection }
{ { ANGLes } }
{ { DIHEdrals } }
{ { IMPRoper-dihedrals } }
{ { CONNectivity } }
Function
The DELEte ATOM option deletes selected atoms and all references
to them in PSF. NOTE: THIS WILL CHANGE THE ATOM NUMBERING.
Note: If PERT is currently in use, this command only affects the active
(lambda=1) PSF. The reference PSF (lambda=0) is only modified by the PERT
command.
For the internal energy terms, any entry that has an atom selected
in both atom selections will be deleted. Note, if an atom is selected in
both atom selections, all connections to this atom will be deleted,
except for bonds. For a bond to be deleted, one of its atoms must
appear in each of the atom selections. The CONN (connectivity) option
will delete all bond, angles, dihedrals, and improper dihedrals.
This option avoids the necessity of running the DELEte command four times
when one wishes to break some connectivity.
The SORT option performs an optional sorting of the PSF after the
deleted atoms have been mapped out.
Delete atoms or energy terms in the structure
[Syntax DELEte terms in structure file]
DELEte { ATOMs atom-selection } [SORT]
{ }
{ { BONDs } double-atom-selection }
{ { ANGLes } }
{ { DIHEdrals } }
{ { IMPRoper-dihedrals } }
{ { CONNectivity } }
Function
The DELEte ATOM option deletes selected atoms and all references
to them in PSF. NOTE: THIS WILL CHANGE THE ATOM NUMBERING.
Note: If PERT is currently in use, this command only affects the active
(lambda=1) PSF. The reference PSF (lambda=0) is only modified by the PERT
command.
For the internal energy terms, any entry that has an atom selected
in both atom selections will be deleted. Note, if an atom is selected in
both atom selections, all connections to this atom will be deleted,
except for bonds. For a bond to be deleted, one of its atoms must
appear in each of the atom selections. The CONN (connectivity) option
will delete all bond, angles, dihedrals, and improper dihedrals.
This option avoids the necessity of running the DELEte command four times
when one wishes to break some connectivity.
The SORT option performs an optional sorting of the PSF after the
deleted atoms have been mapped out.
Top
RENAme - rename portions of the current PSF
[SYNTAX RENAme structure file elements]
RENAme is invoked only from the main command parser and it
includes the working PSF. Its syntax is;
RENAme { SEGId } new-name atom-selection
{ RESId }
{ RESN }
{ ATOM }
Any atoms selected will have the corresponding ID modified.
There is a check for duplicate SEGIDs, RESIDs, and atom names, but it
wont stop you if BOMLEV is negative. Renaming ST2 will not change their
status (except in the setup for SHAKE, which will be fixed soon).
RENAme - rename portions of the current PSF
[SYNTAX RENAme structure file elements]
RENAme is invoked only from the main command parser and it
includes the working PSF. Its syntax is;
RENAme { SEGId } new-name atom-selection
{ RESId }
{ RESN }
{ ATOM }
Any atoms selected will have the corresponding ID modified.
There is a check for duplicate SEGIDs, RESIDs, and atom names, but it
wont stop you if BOMLEV is negative. Renaming ST2 will not change their
status (except in the setup for SHAKE, which will be fixed soon).
Top
Joining Two Adjacent Segments
For some operations, it is convenient to be able to join
two adjacent segments together. This process has no effect on the
energy terms, but just reorganizes naming and grouping of atoms
into segments. This is especially useful with IMAGES so that
all images in the PSF are identified only as a single segment.
Syntax:
JOIN first_segment [second_segment] [ RENUmber ]
The second segment must follow the first sequentially in the
PSF. There is no checking for duplicate residue identifiers. The
RENUmber option sets the resid for each residue of the composite
segment to the relative index in that segment (just as it would have
during a generate command). If only a single segment is specified
with the RENUmber option, then the resid's of this segment will be
numbered sequentially.
Joining Two Adjacent Segments
For some operations, it is convenient to be able to join
two adjacent segments together. This process has no effect on the
energy terms, but just reorganizes naming and grouping of atoms
into segments. This is especially useful with IMAGES so that
all images in the PSF are identified only as a single segment.
Syntax:
JOIN first_segment [second_segment] [ RENUmber ]
The second segment must follow the first sequentially in the
PSF. There is no checking for duplicate residue identifiers. The
RENUmber option sets the resid for each residue of the composite
segment to the relative index in that segment (just as it would have
during a generate command). If only a single segment is specified
with the RENUmber option, then the resid's of this segment will be
numbered sequentially.