# fix momentum command

## Syntax

    fix ID group-ID momentum N keyword values ...

- ID, group-ID are documented in fix command
- momentum = style name of this fix command
- N = adjust the momentum every this many timesteps one or more keyword/value pairs may be appended
- keyword = linear or angular

    linear values = xflag yflag zflag
      xflag,yflag,zflag = 0/1 to exclude/include each dimension
    angular values = none

## Examples

    fix 1 all momentum 1 linear 1 1 0
    fix 1 all momentum 100 linear 1 1 1 angular

## Description

Zero the linear and/or angular momentum of the group of atoms every N timesteps by adjusting the velocities of the atoms. One (or both) of the linear or angular keywords must be specified.

If the linear keyword is used, the linear momentum is zeroed by subtracting the center-of-mass velocity of the group from each atom. This does not change the relative velocity of any pair of atoms. One or more dimensions can be excluded from this operation by setting the corresponding flag to 0.

If the angular keyword is used, the angular momentum is zeroed by subtracting a rotational component from each atom.

This command can be used to insure the entire collection of atoms (or a subset of them) does not drift or rotate during the simulation due to random perturbations (e.g. `fix langevin` thermostatting).

Note that the velocity command can be used to create initial velocities with zero aggregate linear and/or angular momentum.

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during `energy minimization`.

## Restrictions

> none

## Related commands

`fix recenter`, velocity

Default: none