\(\renewcommand{\AA}{\text{Å}}\)

# fix lb/momentum command

## Syntax

    fix ID group-ID lb/momentum nevery keyword values ...

- ID, group-ID are documented in the fix command

- lb/momentum = style name of this fix command

- nevery = adjust the momentum every this many timesteps

- zero or more keyword/value pairs may be appended

- keyword = linear

      linear values = xflag yflag zflag
        xflag,yflag,zflag = 0/1 to exclude/include each dimension.

## Examples

    fix 1 sphere lb/momentum
    fix 1 all lb/momentum linear 1 1 0

## Description

This fix is based on the fix momentum command, and was created to be used in place of that command, when a lattice-Boltzmann fluid is present.

Zero the total linear momentum of the system, including both the atoms specified by group-ID and the lattice-Boltzmann fluid every nevery timesteps. If there are no atoms specified by group-ID only the fluid momentum is affected. This is accomplished by adjusting the particle velocities and the fluid velocities at each lattice site.

Note

This fix only considers the linear momentum of the system.

By default, the subtraction is performed for each dimension. This can be changed by specifying the keyword linear, along with a set of three flags set to 0/1 in order to exclude/ include the corresponding dimension.

## 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

Can only be used if a lattice-Boltzmann fluid has been created via the fix lb/fluid command, and must come after this command.

This fix is part of the LATBOLTZ package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

## Related commands

fix momentum, fix lb/fluid

## Default

Zeros the total system linear momentum in each dimension.