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fort.79 File

The fort.79 file reports the parabolic extrapolation procedure used by ReaxFF to optimize force-field parameters during a force-field optimization run.

It provides the mathematical justification for the optimized parameter values that appear in fort.13 and are subsequently written back into the force field.

Purpose

  • Fit a parabola through three trial parameter values and their total errors
  • Decide whether interpolation, extrapolation, or no update is appropriate
  • Select a new parameter value subject to physical and numerical limits

This file is generated once per parameter listed in the params file.

Optimization Logic

For each parameter, ReaxFF evaluates the total force-field error for:

  1. Lower-bound parameter value
  2. Upper-bound parameter value
  3. Original parameter value

These three points are used to fit a parabola:

[ E(x) = a x^2 + b x + c ]

Decision logic:

  • a > 0 → valid minimum exists
  • Interpolate or extrapolate to the minimum
  • Respect parext extrapolation limits
  • a < 0 → hill-shaped parabola
  • Extrapolation is unsafe
  • Use the best of the three tested values

Example: fort.79 Output

Parameter 1 1 1

Values used for parameter 1 1 1
0.4500000000E+02 0.5500000000E+02 0.5000000000E+02

Differences found
0.9152086304E+02 0.9158925608E+02 0.9164489675E+02

Parabol: a= -0.3593487269E-02 b= 0.3661880309E+00 c= 0.8231921337E+02

Minimum of the parabol 0.5095162492E+02
Difference belonging to minimum of parabol 0.9164814892E+02

New parameter value 0.4500000000E+02
Difference belonging to new parameter value 0.9152086100E+02

Interpretation

  • a < 0 → hill-shaped parabola
  • Extrapolation rejected
  • Best existing value retained

Parameter 1 2 1

Values used for parameter 1 2 1
0.9742293000E+01 0.9939107000E+01 0.9840700000E+01

Differences found
0.9160380360E+02 0.9143296226E+02 0.9151752206E+02

Parabol: a= 0.8889634892E-01 b= -0.2617639062E+01 c= 0.1086682558E+03

Minimum of the parabol 0.1472298410E+02
Difference belonging to minimum of parabol 0.8939852465E+02

New parameter value 0.9988802535E+01
Difference belonging to new parameter value 0.9139091180E+02

Interpretation

  • a > 0 → valid minimum exists
  • Optimal value lies outside allowed extrapolation range
  • Parameter clipped to nearest allowed value

Relation to Other Files

File Role
params Defines which parameters are optimized
fort.13 Total error for each trial
fort.79 Parabolic fit and optimization decision
fort.99 Breakdown of error contributions

Practical Notes

  • fort.79 explains why a parameter was updated (or not)
  • Large extrapolations often signal:
  • Poorly conditioned training sets
  • Overly large search intervals
  • Strong parameter correlations
  • This one-parameter approach is robust and reversible, making it suitable for long optimization campaigns

ReaxKit Context

In ReaxKit, fort.79 is especially useful for:

  • Debugging force-field optimization behavior
  • Auditing parameter updates
  • Visualizing optimization trajectories per parameter