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# compute momentum command

## Syntax

    compute ID group-ID momentum

- ID, group-ID are documented in compute command

- momentum = style name of this compute command

## Examples

    compute 1 all momentum

## Description

Define a computation that calculates the translational momentum p of a group of particles. It is computed as the sum \(\vec{p} = \sum_i m_i \cdot \vec{v}_i\) over all particles in the compute group, where m and v are the mass and velocity vector of the particle, respectively.

## Output info

This compute calculates a global vector (the summed momentum) of length 3. This value can be used by any command that uses a global vector value from a compute as input. See the Howto output page for an overview of LAMMPS output options.

The vector value calculated by this compute is “extensive”. The vector value will be in mass*velocity units.

## Restrictions

This compute is part of the EXTRA-COMPUTE package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

## Related commands

## Default

none