# fix smd/setvel command

## Syntax

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

-   ID, group-ID are documented in [fix](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

``` LAMMPS
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](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](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](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](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](Howto_output). 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](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](Build_package) page
for more info.

## Related commands

none

## Default

none