compute pair command — LAMMPS 15 May 2015 version documentation

# compute pair command

## Syntax

    compute ID group-ID pair pstyle 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
- evalue = epair or evdwl or ecoul or blank (optional setting)

## Examples

    compute 1 all pair gauss
    compute 1 all pair lj/cut/coul/cut ecoul
    compute 1 all pair reax

## Description

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

The evalue setting is optional; it may be left off the command. 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 specified as epair or left out, 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 specfied as evdwl or ecoul, then just that portion of the energy is stored as a global scalar.

Some pair styles tally additional quantities, e.g. a breakdown of potential energy into a dozen or so components is tallied by the pair_style reax commmand. These values (1 or more) are stored as a global vector by this compute. See the doc 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 >= 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 this section 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 doc page for the pair style for details.

## Restrictions

> none

## Related commands

compute pe

## Default

The default for evalue is epair.