# fix wall/reflect/stochastic command

## Syntax

    fix ID group-ID wall/reflect/stochastic rstyle seed face args ... keyword value ...

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

-   wall/reflect/stochastic = style name of this fix command

-   rstyle = diffusive or maxwell or ccl

-   seed = random seed for stochasticity (positive integer)

-   one or more face/args pairs may be appended

-   face = *xlo* or *xhi* or *ylo* or *yhi* or *zlo* or *zhi*

        args = pos temp velx vely velz accomx accomy accomz
          pos = EDGE or constant
            EDGE = current lo or hi edge of simulation box
            constant = number like 0.0 or 30.0 (distance units)
          temp = wall temperature (temperature units)
          velx,vely,velz = wall velocity in x,y,z directions (velocity units)
          accomx,accomy,accomz = accommodation coeffs in x,y,z directions (unitless)
            not specified for rstyle = diffusive
            single accom coeff specified for rstyle maxwell
            all 3 coeffs specified for rstyle cll

-   zero or more keyword/value pairs may be appended

-   keyword = *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 zwalls all wall/reflect/stochastic diffusive 23424 zlo EDGE 300 0.1 0.1 0 zhi EDGE 200 0.1 0.1 0
fix ywalls all wall/reflect/stochastic maxwell 345533 ylo 5.0 300 0.1 0.0 0.0 0.8 yhi 10.0 300 0.1 0.0 0.0 0.8
fix xwalls all wall/reflect/stochastic cercignanilampis 2308 xlo 0.0 300 0.0 0.1 0.9 0.8 0.7 xhi EDGE 300 0.0 0.1 0 0.9 0.8 0.7 units box
```

## Description

Bound the simulation with one or more walls which reflect particles in
the specified group when they attempt to move through them.

Reflection means that if an atom moves outside the wall on a timestep
(e.g. due to the [fix nve](fix_nve) command), then it is put back inside
the wall with a changed velocity.

This fix models treats the wall as a moving solid boundary with a finite
temperature, which can exchange energy with particles that collide with
it. This is different than the simpler [fix
wall/reflect](fix_wall_reflect) command which models mirror reflection.
For this fix, the post collision velocity of each particle is treated
stochastically. The randomness can come from many sources: thermal
motion of the wall atoms, surface roughness, etc. Three stochastic
reflection models are currently implemented.

For rstyle *diffusive*, particles are reflected diffusively. Their
velocity distribution corresponds to an equilibrium distribution of
particles at the wall temperature. No accommodation coefficients are
specified.

For rstyle *maxwell*, particle reflection is Maxwellian which means
partially diffusive and partially specular ([Maxwell](Maxwell)). A
single accommodation coeff is specified which must be between 0.0 and
1.0 inclusive. It determines the fraction of the collision which is
diffusive versus specular. An accommodation coefficient of 1.0 is fully
diffusive; a coefficient of 0.0 is fully specular.

For rstyle *cll*, particle collisions are computed by the
Cercignani/Lampis model. See [CL](CL) and [To](To) for details. Three
accommodations coefficient are specified. Each must be between 0.0 and
1.0 inclusive. Two are velocity accommodation coefficients; one is a
normal kinetic energy accommodation. The normal coeff is the one
corresponding to the normal of the wall itself. For example if the wall
is *ylo* or *yhi*, *accomx* and *accomz* are the tangential velocity
accommodation coefficients, and *accomy* is the normal kinetic energy
accommodation coefficient.

The optional *units* keyword determines the distance units used to
define a wall position. 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.

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

## Restrictions

This fix has the same limitations as the [fix
wall/reflect](fix_wall_reflect) command. Any dimension (xyz) that has a
wall must be non-periodic. It should not be used with rigid bodies such
as those defined by the [fix rigid](fix_rigid) command. The wall
velocity must lie on the same plane as the wall itself.

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

## Related commands

[fix wall/reflect](fix_wall_reflect)

## Default

The default for the units keyword is lattice.

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

::: {#Maxwell}
**(Maxwell)** J.C. Maxwell, Philos. Tans. Royal Soc. London, 157: 49-88
(1867).
:::

::: {#CL}
**(Cercignani)** C. Cercignani and M. Lampis. Trans. Theory Stat. Phys.
1, 2, 101 (1971).
:::

::: {#To}
**(To)** Q.D. To, V.H. Vu, G. Lauriat, and C. Leonard. J. Math. Phys.
56, 103101 (2015).
:::