Skip to content

koppel2 file — Linking force field parameters during optimization

The koppel2 file allows users to link multiple force field parameters during a ReaxFF force field optimization.

Linked parameters are constrained to retain identical values throughout the optimization procedure. This is useful when different parameter entries are physically equivalent or should remain consistent by design.

Purpose of the koppel2 file

During optimization, ReaxFF normally treats each parameter listed in params independently. The koppel2 file overrides this behavior by:

  • Defining groups of linked parameters
  • Enforcing identical values for all parameters in each group
  • Reducing the dimensionality of the optimization problem
  • Improving stability and physical consistency

File handling

  • The koppel2 file is copied by the exe script to fort.23
  • ReaxFF reads parameter-linking instructions exclusively from fort.23
  • The filename koppel2 itself is not read directly by ReaxFF

File format

The koppel2 file consists of link blocks.

Each block contains: 1. A reference parameter identifier 2. The number of parameters linked to it 3. A list of the linked parameter identifiers

Identifier format

Parameter identifiers follow the same convention as in the params file:

<section> <type> <parameter>

Using fixed-width formatting:

4i3

Example 2.17: koppel2 file

5 1 7 5   ! Reference parameter; number of links

5 2 7
<!-- -->
5 3 7
<!-- -->
5 4 7    ! Parameters linked to parameter 5 1 7
<!-- -->
5 5 7
5 6 7

Interpretation of the example

The first line defines the reference parameter:

5 1 7

This corresponds to: - Section: 5 → valence angle parameters - Type: 1 - Parameter: 7

The second integer (5) indicates that five additional parameters are linked to this reference.

Each subsequent line defines one linked parameter:

  • 5 2 7
  • 5 3 7
  • 5 4 7
  • 5 5 7
  • 5 6 7

All six parameters (the reference plus five linked parameters) are forced to share the same value during optimization.

Typical use cases

  • Enforcing symmetry between chemically equivalent interactions
  • Linking parameters across multiple atom types
  • Reducing overfitting in force field optimization
  • Maintaining physical consistency in large parameter sets

Interaction with other optimization files

  • params defines which parameters are optimized and how
  • koppel2 defines which optimized parameters are linked
  • trainset.in defines what data is fitted
  • control controls optimization behavior (parsca, parext)

Together, these files define the complete ReaxFF force field fitting workflow.

Summary

  • koppel2 links force field parameters during optimization
  • All linked parameters retain identical values
  • Uses the same parameter identifier convention as params
  • Copied to fort.23 and read by ReaxFF
  • Essential for constrained and physically consistent force field fitting