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### 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. 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 USER-LB package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

Related commands:

fix momentum, fix lb/fluid

Default:

Zeros the total system linear momentum in each dimension.