ReaxFF File Reference Index

This section of the documentation provides a structured, tool-agnostic reference to ReaxFF input and output files.

It is intended to help users understand how ReaxFF simulations are configured, executed, and analyzed, independently of any specific tooling (including ReaxKit), although ReaxKit parsers and analyzers often build directly on these files.

Provenance note
This Markdown documentation was created on February 2, 2026, based on the Reax_user_manual_June_2017_full_version.pdf.

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How this reference is organized

The documentation is split into two main parts:

• Input files — define what is simulated and how it is run
• Output files — contain trajectories, energies, diagnostics, and optimization data

Each file has its own dedicated page describing: - purpose - format - when it is produced or required - common usage notes

This reference focuses on file purpose and format, not on ReaxKit-specific workflows.

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Input files

ReaxFF input files define what is simulated and how it is run. A full list of them are at input_files/README.md.

Core (mandatory)

• geo — system geometry
• ffield — force-field parameters
• control — run-control settings
• exe — execution script

Optional simulation control

• tregime.in — temperature regimes
• vregime.in — cell / volume control
• eregime.in — electric-field control
• addmol.bgf — molecule insertion
• charges — fixed charges
• vels — MD restart

Force-field optimization

• iopt — optimization toggle
• trainset.in — training-set definition
• params — optimizable parameters
• koppel2 — parameter linking

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Output files

ReaxFF output files contain trajectories, energies, diagnostics, and force-field optimization data.

• xmolout — atomic trajectories
• fort.7 / fort.8 — connectivity and charges
• fort.57 — MM minimization report
• fort.58 — partial MM energies
• fort.71 — MD energy, temperature, pressure
• fort.73 — MD energy contributions
• fort.13 — total force-field error
• fort.79 — parabolic extrapolation
• fort.99 — detailed cost-function report

Additional output files are summarized in the output_files/README.md.

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Included reference manuals

In addition to the Markdown documentation, this directory contains the original ReaxFF user manuals:

• Reax_user_manual_June_2017_full_version.pdf
  The complete historical ReaxFF manual.

• Reax_user_manual_June_2017_trimmed_version.pdf
  A trimmed version with input/output file descriptions removed.

Recommended reading path

For new users:

1. Start with this Markdown reference to understand ReaxFF input and output files.
2. Use the trimmed PDF for:
3. ReaxFF concepts and theory
4. potential functions
5. program structure
6. performance considerations
7. supported execution environments
8. the e-ReaxFF method
9. background literature

Use the full PDF only if historical or low-level detail is required.

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When to use this reference

Use this section if you want to:

• Understand raw ReaxFF file formats
• Interpret ReaxFF output without post-processing tools
• Debug simulations or force-field optimization runs
• Learn how different ReaxFF files interact
• Cross-check ReaxKit results against native ReaxFF output

Where to start: - New simulations → mandatory input files - Force-field training → optimization files - Analysis workflows → output files

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Notes

• File availability depends on run type (MM, MD, FF optimization).
• Output frequency is typically controlled by keywords in control.
• Many output files are overwritten unless append behavior is enabled.

This page serves as the single entry point for all ReaxFF file documentation.