# fix wall/piston command

## Syntax

    fix ID group-ID wall/piston face ... keyword value ...

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

-   wall/piston = style name of this fix command

-   face = *zlo*

-   zero or more keyword/value pairs may be appended

-   keyword = *pos* or *vel* or *ramp* or *temp* or *units*

        *pos* args = z
          z = z coordinate at which the piston begins (distance units)
        *vel* args = vz
          vz = final velocity of the piston (velocity units)
        *ramp* = use a linear velocity ramp from 0 to vz
        *temp* args = target damp seed extent
          target = target velocity for region immediately ahead of the piston
          damp = damping parameter (time units)
          seed = random number seed for langevin kicks
          extent = extent of thermostatted region (distance units)
        *units* value = *lattice* or *box*
          *lattice* = the wall position is defined in lattice units
          *box* = the wall position is defined in simulation box units

## Examples

``` LAMMPS
fix xwalls all wall/piston zlo
fix walls all wall/piston zlo pos 1.0 vel 10.0 units box
fix top all wall/piston zlo vel 10.0 ramp
```

## Description

Bound the simulation with a moving wall which reflect particles in the
specified group and drive the system with an effective infinite-mass
piston capable of driving shock waves.

A momentum mirror technique is used, which means that if an atom (or the
wall) moves such that an atom is outside the wall on a timestep by a
distance delta (e.g. due to [fix nve](fix_nve)), then it is put back
inside the face by the same delta, and the velocity relative to the
moving wall is flipped in z. For instance, a stationary particle hit
with a piston wall with velocity vz, will end the timestep with a
velocity of 2\*vz.

Currently the *face* keyword can only be *zlo*. This creates a piston
moving in the positive z direction. Particles with z coordinate less
than the wall position are reflected to a z coordinate greater than the
wall position. If the piston velocity is vpz and the particle velocity
before reflection is vzi, the particle velocity after reflection is
-vzi + 2\*vpz.

The initial position of the wall can be specified by the *pos* keyword.

The final velocity of the wall can be specified by the *vel* keyword

The *ramp* keyword will cause the wall/piston to adjust the velocity
linearly from zero velocity to *vel* over the course of the run. If the
*ramp* keyword is omitted then the wall/piston moves at a constant
velocity defined by *vel*.

The *temp* keyword will cause the region immediately in front of the
wall/piston to be thermostatted with a Langevin thermostat. This region
moves with the piston. The damping and kicking are measured in the
reference frame of the piston. So, a temperature of zero would mean all
particles were moving at exactly the speed of the wall/piston.

The *units* keyword determines the meaning of the distance units used to
define a wall position, but only when a numeric constant is used.

A *box* value selects standard distance units as defined by the
[units](units) command, e.g. Angstroms for units = real or metal. A
*lattice* value means the distance units are in lattice spacings. The
[lattice](lattice) command must have been previously used to define the
lattice spacings.

------------------------------------------------------------------------

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

No information about this fix is written to [binary restart
files](restart). None of the [fix_modify](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](Howto_output). No
parameter of this fix can be used with the *start/stop* keywords of the
[run](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

This fix style is part of the SHOCK package. It is only enabled if
LAMMPS was built with that package. See the [Build
package](Build_package) page for more info.

The face that has the wall/piston must be boundary type \'s\'
(shrink-wrapped). The opposing face can be any boundary type other than
periodic.

A wall/piston should not be used with rigid bodies such as those defined
by a \"fix rigid\" command. This is because the wall/piston displaces
atoms directly rather than exerting a force on them.

## Related commands

[fix wall/reflect](fix_wall) command, [fix
append/atoms](fix_append_atoms) command

## Default

The keyword defaults are pos = 0, vel = 0, units = lattice.