# fix smd/move_tri_surf command

## Syntax

``` LAMMPS
fix ID group-ID smd/move_tri_surf keyword
```

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

-   smd/move_tri_surf keyword = style name of this fix command

-   keyword = *\*LINEAR* or *\*WIGGLE* or *\*ROTATE*

        *\*LINEAR* args = Vx Vy Vz
           Vx,Vy,Vz = components of velocity vector (velocity units), any component can be specified as NULL
        *\*WIGGLE* args = Vx Vy Vz max_travel
           vx,vy,vz = components of velocity vector (velocity units), any component can be specified as NULL
           max_travel = wiggle amplitude
        *\*ROTATE* args = Px Py Pz Rx Ry Rz period
           Px,Py,Pz = origin point of axis of rotation (distance units)
           Rx,Ry,Rz = axis of rotation vector
           period = period of rotation (time units)

## Examples

``` LAMMPS
fix 1 tool smd/move_tri_surf *LINEAR 20 20 10
fix 2 tool smd/move_tri_surf *WIGGLE 20 20 10
fix 2 tool smd/move_tri_surf *ROTATE 0 0 0 5 2 1
```

## Description

This fix applies only to rigid surfaces read from .STL files via fix
[smd/wall_surface](fix_smd_wall_surface) . It updates position and
velocity for the particles in the group each timestep without regard to
forces on the particles. The rigid surfaces can thus be moved along
simple trajectories during the simulation.

The *\*LINEAR* style moves particles with the specified constant
velocity vector V = (Vx,Vy,Vz). This style also sets the velocity of
each particle to V = (Vx,Vy,Vz).

The *\*WIGGLE* style moves particles in an oscillatory fashion.
Particles are moved along (vx, vy, vz) with constant velocity until a
displacement of max_travel is reached. Then, the velocity vector is
reversed. This process is repeated.

The *\*ROTATE* style rotates particles around a rotation axis R =
(Rx,Ry,Rz) that goes through a point P = (Px,Py,Pz). The period of the
rotation is also specified. This style also sets the velocity of each
particle to (omega cross Rperp) where omega is its angular velocity
around the rotation axis and Rperp is a perpendicular vector from the
rotation axis to the particle.

See [this PDF guide](PDF/MACHDYN_LAMMPS_userguide.pdf)\_ to using Smooth
Mach Dynamics in LAMMPS.

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

Currently, no part of MACHDYN supports restarting nor minimization. This
fix has no outputs.

## 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

[smd/triangle_mesh_vertices](compute_smd_triangle_vertices),
[smd/wall_surface](fix_smd_wall_surface)

## Default

none