compute pair command — LAMMPS documentation

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# compute pair command

## Syntax

    compute ID group-ID pair pstyle [nstyle] [evalue]

- ID, group-ID are documented in compute command

- pair = style name of this compute command

- pstyle = style name of a pair style that calculates additional values

- nsub = n-instance of a sub-style, if a pair style is used multiple times in a hybrid style

- evalue = epair or evdwl or ecoul or blank (optional)

## Examples

    compute 1 all pair gauss
    compute 1 all pair lj/cut/coul/cut ecoul
    compute 1 all pair tersoff 2 epair
    compute 1 all pair reaxff

## Description

Define a computation that extracts additional values calculated by a pair style, and makes them accessible for output or further processing by other commands.

Note

The group specified for this command is ignored.

The specified pstyle must be a pair style used in your simulation either by itself or as a sub-style in a pair_style hybrid or hybrid/overlay command. If the sub-style is used more than once, an additional number nsub has to be specified in order to choose which instance of the sub-style will be used by the compute. Not specifying the number in this case will cause the compute to fail.

The evalue setting is optional. All pair styles tally a potential energy epair which may be broken into two parts: evdwl and ecoul such that epair = evdwl + ecoul. If the pair style calculates Coulombic interactions, their energy will be tallied in ecoul. Everything else (whether it is a Lennard-Jones style van der Waals interaction or not) is tallied in evdwl. If evalue is blank or specified as epair, then epair is stored as a global scalar by this compute. This is useful when using pair_style hybrid if you want to know the portion of the total energy contributed by one sub-style. If evalue is specified as evdwl or ecoul, then just that portion of the energy is stored as a global scalar.

Note

The energy returned by the evdwl keyword does not include tail corrections, even if they are enabled via the pair_modify command.

Some pair styles tally additional quantities, e.g. a breakdown of potential energy into 14 components is tallied by the pair_style reaxff command. These values (1 or more) are stored as a global vector by this compute. See the page for individual pair styles for info on these values.

## Output info

This compute calculates a global scalar which is epair or evdwl or ecoul. If the pair style supports it, it also calculates a global vector of length \(\ge\) 1, as determined by the pair style. These values can be used by any command that uses global scalar or vector values from a compute as input. See the Howto output doc page for an overview of LAMMPS output options.

The scalar and vector values calculated by this compute are “extensive”.

The scalar value will be in energy units. The vector values will typically also be in energy units, but see the page for the pair style for details.

## Restrictions

> none

## Related commands

compute pe, compute bond, fix pair

## Default

The keyword defaults are evalue = epair, nsub = 0.