# compute group/group command

## Syntax

    compute ID group-ID group/group general_keyword general_values group2-ID keyword value ...

- ID, group-ID are documented in compute command
- group/group = style name of this compute command
- general_keywords general_values are documented in compute
- group2-ID = group ID of second (or same) group
- zero or more keyword/value pairs may be appended
- keyword = pair or boundary

    pair value = yes or no
    boundary value = yes or no

## Examples

    compute 1 lower group/group upper
    compute mine fluid group/group wall

## Description

Define a computation that calculates the total energy and force interaction between two groups of atoms: the compute group and the specified group2. The two groups can be the same.

If the pair keyword is set to yes, which is the default, then the the interaction energy will include a pair component which is defined as the pairwise energy between all pairs of atoms where one atom in the pair is in the first group and the other is in the second group. Likewise, the interaction force calculated by this compute will include the force on the compute group atoms due to pairwise interactions with atoms in the specified group2.

This compute does not calculate any bond interactions between atoms in the two groups.

The pairwise contributions to the group-group interactions are calculated by looping over a neighbor list.

## Output info

This compute calculates a global scalar (the energy) and a global vector of length 3 (force), which can be accessed by indices 1-3. 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 LIGGGHTS(R)-PUBLIC output options.

Both the scalar and vector values calculated by this compute are “extensive”. The scalar value will be in energy units. The vector values will be in force units.

## Restrictions

Related commands: none

## Default

The option defaults are pair = yes, kspace = no, and boundary = yes.

Bogusz et al, J Chem Phys, 108, 7070 (1998)