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### compute group/group command

Syntax:

    compute ID group-ID group/group group2-ID

- ID, group-ID are documented in compute command
- group/group = style name of this compute command
- group2-ID = group ID of second (or same) group

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. The interaction energy 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 is the force on the compute group atoms due to pairwise interactions with atoms in the specified group2.

The energy and force are calculated by looping over a neighbor list of pairwise interactions. Thus it can be inefficient to compute this quantity too frequently.

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 LAMMPS 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:

Only pairwise interactions, as defined by the pair_style command, are included in this calculation. Bond (angle, dihedral, etc) interactions between atoms in the two groups are not included. Long-range interactions due to a kspace_style command are also not included. Not all pair potentials can be evaluated in a pairwise mode as required by this compute. For example, 3-body potentials, such as Tersoff and Stillinger-Weber cannot be used. EAM potentials for metals only include the pair potential portion of the EAM interaction, not the embedding term.

Related commands: none

Default: none