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iopt file — Force field optimization switch

The iopt file controls whether ReaxFF runs in normal simulation mode or force field optimization mode.

This file exists because the force field optimization routines are implemented as an external shell around the core ReaxFF code. As a result, the standard control file cannot be used to switch between normal and optimization runs.

Purpose of the iopt file

  • Acts as a mode selector for ReaxFF
  • Read by the external optimization shell
  • Copied by the exe script to fort.20
  • Determines whether force field optimization logic is activated

File format

The iopt file contains exactly one integer, using format i3:

<value>

Allowed values

Value Meaning
0 Normal ReaxFF run
1 Force field optimization run

Example

1

This configuration instructs ReaxFF to execute a force field optimization run.

Behavior when missing

Although the iopt file is officially mandatory, most workflows do not require the user to create it manually.

  • If the iopt file is missing, the exe script:
  • Automatically creates fort.20
  • Assigns a default value of 0
  • This results in a normal (non-optimization) run

Relation to the exe script

  • The exe script is responsible for:
  • Copying iopt → fort.20
  • Ensuring a valid default exists if iopt is absent
  • Users typically do not need to interact with iopt directly unless performing force field optimization

Summary

  • iopt is a single-value control file
  • It switches ReaxFF between normal and optimization modes
  • Defaults to 0 (normal run) if not explicitly provided
  • Required for advanced force field fitting workflows