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# fix reaxff/bonds command

Accelerator Variants: reaxff/bonds/kk

## Syntax

    fix ID group-ID reaxff/bonds Nevery filename

- ID, group-ID are documented in fix command

- reax/bonds = style name of this fix command

- Nevery = output interval in timesteps

- filename = name of output file

## Examples

    fix 1 all reaxff/bonds 100 bonds.reaxff

## Description

Write out the bond information computed by the ReaxFF potential specified by pair_style reaxff in the exact same format as the original stand-alone ReaxFF code of Adri van Duin. The bond information is written to filename on timesteps that are multiples of Nevery, including timestep 0. For time-averaged chemical species analysis, please see the fix reaxff/species command.

The specified group-ID is ignored by this fix except for the dump image related functionality (see below).

The format of the output file should be reasonably self-explanatory. The meaning of the column header abbreviations is as follows:

- id = atom id

- type = atom type

- nb = number of bonds

- id_1 = atom id of first bond

- id_nb = atom id of Nth bond

- mol = molecule id

- bo_1 = bond order of first bond

- bo_nb = bond order of Nth bond

- abo = atom bond order (sum of all bonds)

- nlp = number of lone pairs

- q = atomic charge

If the filename ends with “.gz” or some other supported compression format suffix, the output file is written in compressed format. A compressed output file can be significantly smaller than the text version, but will also take longer to write.

Added in version 2Apr2025.

If the filename contains the wildcard character “*”, a new file is created on every timestep there bond information is written. The “*” character is replaced with the timestep value. Note that the fix_modify pad command can be used so that all timestep numbers have the same length by adding leading zeroes (e.g. 00010 for a pad value of 5). The default pad value is 0, i.e. no leading zeroes.

Added in version 11Feb2026.

If the filename is “NULL”, then no output is created. This can be useful when using fix reaxff/bonds in combination with dump image fix keyword to visualize the bonds computed by the ReaxFF force field.

## Dump image info

Added in version 11Feb2026.

Fix reaxff/bonds supports the fix keyword of dump image. The fix will pass geometry information about the bonds computed by the ReaxFF pair style to dump image so that they can be included in the rendered image. Only bonds where both atoms are within the fix group generate graphics objects that are displayed in the dumped images. That group may be a dynamic group.

The color of the bonds is by default that of the atoms when using color styles “type” or “element”. With color style “const” the default value of “white” can be changed using dump_modify fcolor. The transparency is by default fully opaque and can be changed with dump_modify ftrans.

The fflag1 setting of dump image fix determines whether the bonds will be capped with spheres (1) or not (0).

The fflag2 setting allows to adjust diameter of the cylinders for the bonds. By default, a diameter of 0.5 length units will be used.

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to binary restart files. This fix supports the fix_modify pad option. No global or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

Styles with a gpu, intel, kk, omp, or opt suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the Accelerator packages page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Build package page for more info.

You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use the suffix command in your input script.

See the Accelerator packages page for more instructions on how to use the accelerated styles effectively.

## Restrictions

The fix reaxff/bonds command requires that the pair_style reaxff is invoked. This fix is part of the REAXFF package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

To write compressed bond files, you must compile LAMMPS with the `-DLAMMPS_GZIP` option. See the Build settings doc page for details.

## Related commands

pair_style reaxff, fix reaxff/species

## Default

pad = 0