\(\renewcommand{\AA}{\text{Å}}\)

# fix wall/piston command

## Syntax

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

- ID, group-ID are documented in 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

    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), 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 command, e.g. Angstroms for units = real or metal. A lattice value means the distance units are in lattice spacings. The 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. 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

This fix style is part of the SHOCK package. It is only enabled if LAMMPS was built with that package. See the 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 command, fix append/atoms command

## Default

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