\(\renewcommand{\AA}{\text{Å}}\)

# fix smd/setvel command

## Syntax

    fix ID group-ID smd/setvel vx vy vz keyword value ...

- ID, group-ID are documented in fix command

- smd/setvel = style name of this fix command

- vx,vy,vz = velocity component values

- any of vx,vy,vz can be a variable (see below)

- zero or more keyword/value pairs may be appended to args

- keyword = region

      region value = region-ID
        region-ID = ID of region particles must be in to have their velocities set

## Examples

    fix top_velocity top_group smd/setvel 1.0 0.0 0.0

## Description

Set each component of velocity on each particle in the group to the specified values vx,vy,vz, regardless of the forces acting on the particle. This command can be used to impose velocity boundary conditions.

Any of the vx,vy,vz values can be specified as NULL which means do not alter the velocity component in that dimension.

This fix is indented to be used together with a time integration fix.

Any of the 3 quantities defining the velocity components can be specified as an equal-style or atom-style variable, namely vx, vy, vz. If the value is a variable, it should be specified as v_name, where name is the variable name. In this case, the variable will be evaluated each timestep, and its value used to determine the force component.

Equal-style variables can specify formulas with various mathematical functions, and include thermo_style command keywords for the simulation box parameters and timestep and elapsed time. Thus it is easy to specify a time-dependent velocity field.

Atom-style variables can specify the same formulas as equal-style variables but can also include per-atom values, such as atom coordinates. Thus it is easy to specify a spatially-dependent velocity field with optional time-dependence as well.

If the region keyword is used, the particle must also be in the specified geometric region in order to have its velocity set by this command.

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

Currently, no part of MACHDYN supports restarting nor minimization None of the fix_modify options are relevant to this fix.

This fix computes a global 3-vector of forces, which can be accessed by various output commands. This is the total force on the group of atoms. The vector values calculated by this fix are “extensive”.

No parameter of this fix can be used with the start/stop keywords of the run command.

## Restrictions

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

## Related commands

none

## Default

none