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params file — Force field parameter optimization control

The params file specifies which force field parameters are included in a ReaxFF force field optimization and defines how each parameter is searched during optimization.

It tells ReaxFF: - Which parameter to optimize - How far to perturb it during the search - What bounds constrain the optimization

Purpose of the params file

During force field optimization, ReaxFF does not vary all parameters automatically. Instead, it iterates through the parameters listed in params, optimizing them one at a time using a constrained parabolic extrapolation.

For each parameter: 1. ReaxFF perturbs the parameter value 2. Recalculates the cost function (from trainset.in) 3. Fits a parabola to determine an improved value 4. Applies bounds and extrapolation limits

Results of each optimization step are written to fort.79.

File handling

  • The params file is copied by the exe script to fort.21
  • ReaxFF reads optimization instructions exclusively from fort.21
  • The filename params itself is not read directly by ReaxFF

File format

Each non-comment line defines one optimizable force field parameter.

Line format

<section> <type> <parameter> <step> <max> <min>

Using fixed-width formatting:

3i3, 3f8.4

Field definitions

Field Meaning
section Force field section index
type Atom/bond/angle/torsion type index
parameter Parameter index within that type
step Relative perturbation factor
max Maximum allowed value
min Minimum allowed value

Force field section indices

The section integer maps to the corresponding section in the ffield file:

Index Section
1 General parameters
2 Atom parameters
3 Bond parameters
4 Off-diagonal parameters
5 Valence angle parameters
6 Torsion angle parameters
7 Hydrogen bond parameters

Example 2.16: params file

1 1 1 0.100 75.000 25.000   ! 1st general parameter
1 2 1 0.010 10.000  9.000   ! 2nd general parameter
2 1 1 0.001  1.600  1.350   ! section 2, type 1, parameter 1
2 2 1 0.001  0.700  0.600   ! section 2, type 2, parameter 1
2 2 5 0.010  0.020  0.030   ! section 2, type 2, parameter 5
6 3 4 0.050 -3.000 -5.000   ! section 6, type 3, parameter 4

Interpretation of an example line

Consider the final line:

6 3 4 0.050 -3.000 -5.000

This instructs ReaxFF to optimize: - Section: 6 → torsion angle parameters - Type: 3 → specific torsion type - Parameter: 4 → fourth torsion parameter - Search step: ±5% relative perturbation - Bounds: constrained between −5.000 and −3.000

If the starting value in ffield is −4.7435, ReaxFF will: - Increase and decrease the value by 5% - Evaluate the cost function - Perform a parabolic extrapolation - Choose an improved value within the specified bounds

Optimization workflow

  • ReaxFF processes parameters sequentially, top to bottom
  • Each parameter is optimized independently
  • After the final line is processed, the optimization run terminates
  • Detailed extrapolation results are written to fort.79

Interaction with control file

Two control keywords affect parameter optimization behavior:

Keyword Effect
parext Limits how far extrapolation may extend beyond the search interval
parsca Scales the search interval size globally

Because the control file is reread during optimization, these values can be adjusted mid-run to influence long optimizations.

Best practices

  • Start with small step sizes (step ≈ 0.001–0.05)
  • Use tight bounds to prevent unphysical parameters
  • Optimize most sensitive parameters first
  • Monitor fort.79 to verify convergence behavior

Summary

  • params defines which force field parameters are optimized
  • Each line controls one parameter’s search behavior
  • Optimization proceeds sequentially and deterministically
  • Central to reproducible and controlled ReaxFF force field fitting