fix pair command — LAMMPS documentation

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# fix pair command

## Syntax

    fix ID group-ID pair N pstyle name flag ...

- ID, group-ID are documented in fix command

- pair = style name of this fix command

- N = invoke this fix once every N timesteps

- pstyle = name of pair style to extract info from (e.g. eam)

- one or more name/flag pairs can be listed

- name = name of quantity the pair style allows extraction of

- flag = 1 if pair style needs to be triggered to produce data for name, 0 if not

## Examples

    fix request all pair 100 eam rho 0
    fix request all pair 100 amoeba uind 0 uinp 0

## Description

Added in version 15Sep2022.

Extract per-atom quantities from a pair style and store them in this fix so they can be accessed by other LAMMPS commands, e.g. by a dump command or by another fix, compute, or variable command.

These are example use cases:

- extract per-atom density from pair_style eam to a dump file

- extract induced dipoles from pair_style amoeba to a dump file

- extract accuracy metrics from a machine-learned potential to trigger output when a condition is met (see the dump_modify skip command)

The N argument determines how often the fix is invoked.

The pstyle argument is the name of the pair style. It can be a sub-style used in a pair_style hybrid command. If there are multiple sub-styles using the same pair style, then pstyle should be specified as “style:N”, where N is the number of the instance of the pair style you wish monitor (e.g., the first or second). For example, pstyle could be specified as “pace/extrapolation” or “amoeba” or “eam:1” or “eam:2”.

One or more name/flag pairs of arguments follow. Each name is a per-atom quantity which the pair style must recognize as an extraction request. See the doc pages for individual pair_styles to see what fix pair requests (if any) they support.

The flag setting determines whether this fix will also trigger the pair style to compute the named quantity so it can be extracted. If the quantity is always computed by the pair style, no trigger is needed; specify flag = 0. If the quantity is not always computed (e.g. because it is expensive to calculate), then specify flag = 1. This will trigger the quantity to be calculated only on timesteps it is needed. Again, see the doc pages for individual pair_styles to determine which fix pair requests (if any) need to be triggered with a flag = 1 setting.

The per-atom data extracted from the pair style is stored by this fix as either a per-atom vector or array. If there is only one name argument specified and the pair style computes a single value for each atom, then this fix stores it as a per-atom vector. Otherwise a per-atom array is created, with its data in the order of the name arguments.

For example, pair_style amoeba allows extraction of two named quantities: “uind” and “uinp”, both of which are 3-vectors for each atom, i.e. dipole moments. In the example below a 6-column per-atom array will be created. Columns 1-3 will store the “uind” values; columns 4-6 will store the “uinp” values.

    pair_style amoeba
    fix ex all pair 10 amoeba uind 0 uinp 0

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

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix.

As explained above, this fix produces a per-atom vector or array which can be accessed by various output commands. If an array is produced, the number of columns is the sum of the number of per-atom quantities produced by each name argument requested from the pair style.

## Restrictions

none

## Related commands

compute pair

## Default

none