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charges file — Fixed atomic charges for ReaxFF

The charges file defines fixed atomic charges for use in ReaxFF simulations.

When the keyword:

icharg = 5

is set in the control file, ReaxFF disables geometry-dependent charge models and instead reads static charges from the charges input file.

Purpose of the charges file

Using fixed charges can:

  • Improve simulation stability during early equilibration
  • Reduce computational cost compared to dynamic charge models
  • Enable controlled comparisons with non-reactive force fields

However, fixed charges do not adapt during reactions, which may lead to unphysical results in highly reactive simulations.

File handling

  • The charges file is copied by the exe script to fort.26
  • ReaxFF reads fixed charges exclusively from fort.26
  • The filename charges itself is not read directly by ReaxFF

File structure overview

The charges file is organized by molecule types, allowing multiple identical molecules to share the same charge definition.

General layout

  1. Number of molecule types
  2. Molecule index ranges for each type
  3. For each molecule type:
  4. Molecule identifier (comment line)
  5. Number of atoms
  6. Atom number–charge pairs

Example 2.15: charges file

The following example defines fixed charges for a system containing:

  • 5 methane molecules (molecules 1–5)
  • 5 water molecules (molecules 6–10)
2   ! Number of molecule types (format i4)

1 5 6 10   ! Molecule type definition (format 20i4)

Methane   ! Molecule 1 identifier
5         ! Number of atoms in molecule 1

1 -0.4800 ! Atom number; charge (format i4,f10.6)
2  0.1200
3  0.1200
4  0.1200
5  0.1200

Water     ! Molecule 2 identifier
3         ! Number of atoms in molecule 2

1 -0.8200 ! Atom number; charge
2  0.4100
3  0.4100

Keyword and field description

Number of molecule types

2

Specifies how many distinct molecule charge templates are defined.

Molecule type definition

1 5 6 10

Defines which molecule indices correspond to each molecule type:

  • Molecule type 1 → molecules 1–5
  • Molecule type 2 → molecules 6–10

Molecule identifier

A comment-style label identifying the molecule type:

Methane
Water

Used for readability only.

Atom charges

Each atom charge entry specifies:

<atom index> <charge>
  • Atom indices must match the atom ordering in the geometry file
  • Charges are given in elementary charge units (e)

Practical notes

  • The charges format is well suited for systems with repeating molecular motifs
  • Fixed-charge definitions can often be generated by:
  • Running a short ReaxFF simulation with dynamic charges
  • Extracting averaged charges from fort.56
  • Fixed charges may:
  • Improve numerical stability early on
  • Become inaccurate once reactions or charge transfer occur

When to use fixed charges

Recommended - Initial equilibration - Weakly reactive or non-reactive systems - Benchmarking performance

Use with caution - Reactive chemistry - Charge-transfer dominated processes - Long production MD runs

Summary

  • The charges file enables fixed-charge ReaxFF simulations
  • Activated via icharg = 5 in the control file
  • Copied to fort.26 by the exe script
  • Powerful for equilibration and speed, but limited for reactive accuracy