Skip to main content

fitparam (c47b2)

Parameter Fitting Procedure

By Victor Anisimov (
and Alex MacKerell Jr. (; December 2007

FITPARAM is a parameter fitting procedure that is primarily designed to fit
partial atomic charges and atomic polarziabilities based on the Drude
oscillator model to interaction energy data and dipole moments, though it may
be applied to fitting of other parameters (see below). It supports optimization
of multiple parameters for a series of model compounds sharing common parameter
sets. Different weights can be assigned to different target data. Optimized
parameters can be restrained to their corresponding initial values by using
a parabolic penalty function. FITPARAM performs non-linear least square fitting
using the Levenberg-Marquardt algorithm.

* Introduction | Overview of functionality
* Syntax | Syntax of commands
* Keywords | Description of keywords
* Format | File format
* Example | Input examples
* Limitations | Known limitations

Overview of functionality

The primary purpose of FITPARAM is charge derivation using various types of
interaction energy as a target data. In this goal it complements the
functionality of the FITCHARGE module which derives charges solely from
fitting to electrostatic potentials (» fitcharge ). However, FITPARAM
is not limited to charge derivation and it can be used to optimize any sort of
parameters using any sort of relevant target data.

Although implemented in CHARMM the FITPARAM module is in fact a stand-alone
procedure. The user is not required to load topology and generate a molecule
and if loaded this information will be ignored. The command FITPARAM operates
with generic parameters without the awareness whether these are point charges,
atomic polarizabilities or dihedral force constants. Thus, any parameter may
be optimized using FITPARAM based on basic least square fitting.
Correspondingly, calculation of the target property and its gradient has to be
performed separately by external routines. Preparing the necessary scripts
to accomplish this task becomes the user assignment. Despite this obvious
inconvenience, such architectural design is necessary to allow for flexibility
in the target data used, which otherwise would not be accessible for the
parameter optimization. It also makes possible parameter fitting for a variety
of molecules to be performed simultaneously.

In order to start parameter optimization FITPARAM needs an initial guess for
parameters. It reads the parameter values from the input file, performs a
cycle of least square optimization and saves the updated parameters in the
output file. Then FITPARAM executes the external job, which performs target
property and gradient calculations using the current parameter values and
returns the control back to FITPARAM. These iterations are continued until
convergence criterion is satisfied.

Using FITPARAM to optimize charges requires the user to preserve the total
charge of the optimized molecule. This can be accomplished by excluding one
charge from the fitting and computing the value of the excluded charge in the
external procedure as the difference in total charge of the molecule and the
sum of optimized charges. In general it is not important which atomic charge
to exclude from the fitting. The general suggestion is to exclude that charge
which appears least important in reproducing the value of the target data.

Dipole moment information can be provided to FITPARAM in two different ways. A
dipole may be presented in scalar form as the total dipole moment or in vector
form representing three dipole moment components. These two ways of dipole
moment specification are handled independently by FITPARAM. When dealing with
multiple molecules each molecule may be represented by its own total dipole
moment and dipole components. The total dipole moment value may be defined as
the gas-phase experimental value, or QM computed value, or manually defined
value the user is targeting as the result of the fitting. Providing manually
increased (or HF/6-31G* computed) dipole moments is a standard practice when
deriving charges for a non-polarizable model. Providing the dipole moment in
vector form has the purpose of minimizing the angle between the model and
target dipole moment vectors.

Syntax of commands

[SYNTAX FITPARAM - parameter optimization]

[NITEr int] [TOLErance real] [COUNter int]

NEXPeriments int NPARameters int
GUESs int EXPData int PARM int

[EXPWeight int] [RESTraints int]

[ [NDIPoles int] [FVALues int REXternal str [MULTiplicity int] ]

Description of keywords

NITEr - (optional) maximum number of Levenberg-Marquardt (least square)
iterations. Default value NITE=100.
TOLErance - (optional) relative tolerance on the convergence of the
minimized function. Default value TOLE=1.0E-4.
COUNter - (optional) is number of iterations under the tolerance it needs to
converge. Default value COUN=2. Greater values can be used to assure the

NEXPeriments - (mandatory) number of target data. Care must be taken when
handling dipole moments which can be provided in scalar (single value for
total dipole) or vector form (three dipole component values in a row). Three
components of a vector are considered as one target value for NEXP keyword.
Total dipole moment value counts independently as one experimental value. This
is also discussed in the examples section.
NPARameters - (mandatory) number of parameters to optimize.

GUESs - (mandatory) file unit number for initial parameter guess.
EXPData - (mandatory) file unit number for target ("experimental") data.
PARM - (mandatory) file unit number to save optimized parameter values.

EXPWeight - (optional) file unit number for individual weights assigned to
each target value. Default value is 1.0 for each target value.
RESTraints int - (optional) file unit number for individual restraint
factors assigned to each optimized parameter. Default value is 0.0, which
means free fitting without restraints.

Following keywords are exclusive.

ANTOINE - (mandatory if Antoine Constant calculation is expected) switches
on Antoine parameter fitting to find analytical dependence in experimental
vapor pressure - temperature dependence. This information is useful for
calculation of experimental heat of vaporization.


NDIPoles - (mandatory, if dipoles are provided in vector form) number of
dipole moments specified in vector form (X Y Z form). Default value is 0. Note,
NDIP keyword counts only the number of dipoles provided in vector form (X Y Z
components). Correspondingly, NDIP does not count the number of dipoles
provided in scalar form (total dipole moment).
FVALues - file unit number. This file provides function values and gradients
to FITPARAM. The data in this file have to be computed externally using updated
parameter values (which are saved in PARM unit by FITPARAM). FITPARAM needs
current target function values and gradients to perform next parameter
optimization iteration.
REXternal - external procedure provided in the form of a string enclosed in
double quotes. This string will be executed by CHARMM. This external procedure
is in charge of calculating current function values and gradients.
MULTiplicity - (optional) file unit number. The data in this file specify th
multiplicity of the optimized parameters. Default value of multiplicity for
each parameter value is 1. FITPARAM computes the total charge using the
information about charge multiplicity. This data makes no influence on the
progress of parameter optimization and is implemented for debugging purpose
only. For example, three hydrogen atoms in methyl group carrying the same
charge value imply a multiplicity of 3. Correct accounting for charge
multiplicity will help FITPARAM to provide meaningful information about total
charge of the optimized molecule. Non-charge parameters should be assigned
multiplicity 0. This is particularly helpful to when FITPARAM optimizes charges
simultaneously with non-charge parameters.

File format

GUESs unit
Includes NPAR number of lines.
Format: string (up to 20 characters before real number is encountered), real
(recognized by decimal point). Example: "A 17.81671". The string portion of
the data will be printed by FITPARAM as the parameter name. One may use the
string portion to do some useful work for external job, e.g. "set A 17.81671"
would turn the parameter file into a functional CHARMM script file.

EXPD unit
Includes NEXP number of lines.
Format: one real value per string. (Exception is Antoine input file which
contains two real numbers in free format. First value is temperature; the
second one is vapor pressure.) Total dipole moment is defined by one value.
Dipole moment components are specified by three numbers in free format.
1.63 ! dipoleTotal MP2 value
-1.5155 0.1868 -0.5583 ! dipoleXYZ
Comments can be placed after exclamation mark. This information will be
ignored by FITPARAM.

PARM unit
Includes NPAR number of lines.
Format: A20,F16.8
This is an output file which will be created by FITPARAM.

FVAL unit
Includes NEXP + NEXP * NPAR number of lines.
Format: The format is free. This file is created by external job and read by
FITPARAM. The external job creating this file has to take care of the following
requirements. One real number per string is expected for scalar values. Three
numbers are provided for a vector value. First NEXP data are the function
values which are computed for NEXP target data using the current parameter
values. The order of computed values must be the same as in the EXPD file.
Next NPAR * NEXP data are partial derivatives computed in the order of
experimental data (first running index) and parameter data (second running
index). Example:
Where E1, E2, E3 are target (experimental) data; p1 and p2 are optimized
parameters. Derivatives of vector properties are provided by three real
numbers in a row (see example below).

EXPW unit
Includes NEXP number of lines.
Format: one real number in a row.

MULT unit
Includes NPAR number of lines.
Format: one integer number in a row.

REST unit
Includes NPAR number of lines.
Format: one real number in a row.

Input Examples

Two examples given in this section illustrate basic functionality of FITPARAM.
First example covers optimization of Antoine function parameters. This is an
example where the function value and gradient computations are implemented
inside the FITPARAM module so there is no need to call an external procedure.
Therefore this is an exception to the standard use of FITPARAM. This example
is discussed here because of its simplicity and because it is included in

The Antoine function is a simple parametric analytical function which is used
to describe experimental vapor pressure - temperature dependence. It has the
following form: lnP = A + [B / (T + C)], where A,B, and C are fitted
parameters, P - pressure, T- temperature.

Having this function gives the opportunity to compute derivative of pressure
over temperature which is necessary to compute the heat of vaporization of the
pure liquid. Following is the content of the antoine.inp script:

open unit 11 read form name antoine.ini ! initial guess for parameters
open unit 12 read form name antoine.exp ! experimental (target data)
open unit 13 write form name antoine.prm ! storage for optimized parameters
NITE 50 - ! maximum number of iterations
TOLE 0.001 - ! chi^2 convergence threshold
COUN 2 - ! number of consecutive successful steps before convergence
NEXP 8 - ! number of experimental data
NPAR 3 - ! number of parameters to fit (2 or 3 Antoine coefficients)
ANTOINE - ! Antoine coefficient fitting
GUES 11 - ! input: initial guess for parameters
EXPD 12 - ! input: data to fit to
PARM 13 ! output: file to store optimized parameters

The computation starts from opening two input files antoine.ini and
antoine.exp, which contain initial guess for parameters and target experimental
data, respectively. Following is the content of antoine.ini file:
A 17.81671
B 4705.03330
C -60.75000

Three parameters, NEXP=3, will be optimized starting form the above values.

The antoine.exp file contains the following data:
393.15 3.649359
398.15 3.877432
403.15 4.076690
408.15 4.264087
413.15 4.461877
418.15 4.651099
423.15 4.825109
428.15 5.018603

Here we have 8 experimental data, NEXP=8, representing temperature - vapor
pressure data.

Following parameter values are obtained after the execution of the above
A 17.84653417
B 4706.72855119
C -61.45937115

The second example illustrates how to set up FITPARAM calculation for a charge
parameter optimization calculation. The example is based on hydroxyl charge
optimization in ethanol targeting interactions with water, which is a standard
charge derivation procedure in the additive CHARMM force field. In the example
the alkane charges in methyl group are constrained to their standard values.
The methyl group is also kept electro-neutral. Two lone pairs (OLP) are
assigned to oxygen atom in this example; therefore the oxygen atom is
represented by two point charges (qOLP). Note, the central O atom carries zero
charge. The charges to be determined are qC, two qHC, two qOLP, and qHO.
From these variables one should be excluded. This charge should be assigned
manually to maintain a total charge of zero. In the present example we exclude
the methylene group hydrogen atom charge: qH = -1/2 * (2*qOLP + qC + qHO).
Remaining variables define NPARM=3. The lone pairs are equivalent therefore
they contribute as one parameter qOLP. Correspondingly, the initial guess
parameter file "parameters.ini" contains the following data:
set qOLP -0.23
set qHO 0.36
set qC -0.06

Because the qOLP parameter has the multiplicity of 2 we declare this in the
"multiplicity" file:

Here the parameter multiplicity is preceded with the "group" keyword which
indicates the program that an electro-neutral group is being treated. If the
methyl group charges were also included in the fitting, then the
"multiplicity" file would contain the following data:


Here we optimize the charge on methyl hydrogens (which has multiplicity 3),
whereas the charge on methyl carbon has to be computed externally to keep the
CH3 group electro-neutral.

The charges specified in the "parameters.ini" file are taken from the CHARMM22
force field. To restrain the charges to their initial values we specify
"restraints" file using the following restraint factors:

The smaller the number the weaker the restraint. To choose a particular value
one needs to do some experimenting. Next we define the target data in
-4.89 ! C-O-H angle bisector
-2.51 ! along C-O line
-4.80 ! lone-pair position
-4.36 ! along O-H line
1.8 ! dipoleTotal MP2 = 1.6259
-1.5155 0.1868 -0.5583 ! dipoleXYZ

Presented is ethanol - water QM interaction energies and the ethanol dipole.
First four values in the expdata file are interaction energies for the
corresponding four water orientations. The fifth value is the ethanol dipole
moment set to 1.8 Debye. Note, that the MP2 dipole is 1.63. The dipole moment
is purposefully increased to provide an illustrative example that we are free
to define any target data we want our model to reproduce. While we are
experimenting with the magnitude of the total dipole moment we certainly want
to minimize the angle between the target and empirical dipole moment vectors.
Therefore the sixth line of data is the dipole moment specified in vector form.
Overall, we have six target data, NEXP=6, and one dipole vector, NDIP=1.
Weighting of data is performed via the "expweight" file that instructs
FITPARAM of the level of priority assigned to the target data:

Each line in this file applies to corresponding target data in the "expdata"
file. The value 1.0 means a standard weight. The larger the value we specify
the stronger FITPARAM will try to match the target data during the parameter
optimization. One needs to make some empirical adjustments in order to define
the optimal value for the each weighting factor.

The FITPARAM script to run the optimization has the following form:

open unit 11 read form name parameters.ini
open unit 12 read form name expdata
open unit 13 write form name parameters
open unit 14 write form name fvalues
open unit 15 read form name expweight
open unit 16 read form name multiplicity
open unit 17 read form name restraints
NITE 100 - ! max number of iterations
TOLE 0.0001 - ! convergence tolerance
COUN 4 - ! steps to test the convergence
NEXP 6 - ! number of target data
NDIP 1 - ! number of dipole moments among the target data
NPAR 3 - ! number of parameters to optimize
GUES 11 - ! initial guess for parameters
EXPD 12 - ! target data
PARM 13 - ! place to store optimized parameters between loops
FVAL 14 - ! function values and derivatives
REXT "./ ./parameters ./fvalues" - ! script to compute derivatives
EXPW 15 - ! weight of individual target data, optional
MULT 16 - ! parameter multiplicity, optional
REST 17 ! weight of parameter restraint, optional

After reading initial guesses for parameters specified in the "parameters.ini"
file FITPARAM will write updated parameter values in the "parameters" file
(see unit PARM 13). The "parameters" file can be streamed into a CHARMM script
to be run externally to compute energies and gradients using the current
parameter values. This is done with the help of the REXT keyword (which stands
for Run EXTernal process), which tells FITPARAM how to invoke the external
process to compute the function values and derivatives. In this particular
example we invoke the string "./ ./parameters ./fvalues". This says that
computation will be managed by perl script "" which we assume will invoke
individual computations. The perl script takes two arguments. The "parameter"
file contains current parameter values computed by FITPARAM. The "fvalues"
file instructs "" script to save the function values and gradients in
the "fvalues" file, because FITPARAM will be reading it (see FVAL 14 unit)
after "" script finishes its computation. The perl script or other user
selected external routine has to be written individually for each optimization
job. Due to the nature of individual computations and because of the multitude
of possibilities it is difficult to provide one set of rules on how to do this
in the best way. Preparing such script the user should take care about writing
the "fvalues" file in the right format (see above). The data in this file are
rewritten with each optimization cycle. For the example shown above the
"fvalues" file had the following intermediate values:
-1.72300 0.03500 -0.71400
0.89123 -6.56414 -2.47438
0.55868 -4.51683 -1.57192
-0.09208 -0.23845 0.21026

Relating these data to the data in "expdata" file helps to reinforce the
understanding of the format of the "fvalues" file.

Known limitations

The present implementation does not preserve the total charge. Therefore, the
user is responsible to manage this problem in designing the external procedure.
Implementing the charge constraint mechanism is in future development plans.