# Walls

Walls in an MD simulation are typically used to bound particle motion,
i.e. to serve as a boundary condition.

Walls in LAMMPS can be of rough (made of particles) or idealized
surfaces. Ideal walls can be smooth, generating forces only in the
normal direction, or frictional, generating forces also in the
tangential direction.

Rough walls, built of particles, can be created in various ways. The
particles themselves can be generated like any other particle, via the
[lattice](lattice) and [create_atoms](create_atoms) commands, or read in
via the [read_data](read_data) command.

Their motion can be constrained by many different commands, so that they
do not move at all, move together as a group at constant velocity or in
response to a net force acting on them, move in a prescribed fashion
(e.g. rotate around a point), etc. Note that if a time integration fix
like [fix nve](fix_nve) or [fix nvt](fix_nh) is not used with the group
that contains wall particles, their positions and velocities will not be
updated.

-   [fix aveforce](fix_aveforce) - set force on particles to average
    value, so they move together
-   [fix setforce](fix_setforce) - set force on particles to a value,
    e.g. 0.0
-   [fix freeze](fix_freeze) - freeze particles for use as granular
    walls
-   [fix nve/noforce](fix_nve_noforce) - advect particles by their
    velocity, but without force
-   [fix move](fix_move) - prescribe motion of particles by a linear
    velocity, oscillation, rotation, variable

The [fix move](fix_move) command offers the most generality, since the
motion of individual particles can be specified with
[variable](variable) formula which depends on time and/or the particle
position.

For rough walls, it may be useful to turn off pairwise interactions
between wall particles via the [neigh_modify exclude](neigh_modify)
command.

Rough walls can also be created by specifying frozen particles that do
not move and do not interact with mobile particles, and then tethering
other particles to the fixed particles, via a [bond](bond_style). The
bonded particles do interact with other mobile particles.

Idealized walls can be specified via several fix commands. [Fix
wall/gran](fix_wall_gran) creates frictional walls for use with granular
particles; all the other commands create smooth walls.

-   [fix wall/reflect](fix_wall_reflect) - reflective flat walls
-   [fix wall/lj93](fix_wall) - flat walls, with Lennard-Jones 9/3
    potential
-   [fix wall/lj126](fix_wall) - flat walls, with Lennard-Jones 12/6
    potential
-   [fix wall/colloid](fix_wall) - flat walls, with [pair_style
    colloid](pair_colloid) potential
-   [fix wall/harmonic](fix_wall) - flat walls, with repulsive harmonic
    spring potential
-   [fix wall/morse](fix_wall) - flat walls, with Morse potential
-   [fix wall/region](fix_wall_region) - use region surface as wall
-   [fix wall/gran](fix_wall_gran) - flat or curved walls with
    [pair_style granular](pair_gran) potential

The *lj93*, *lj126*, *colloid*, *harmonic*, and *morse* styles all allow
the flat walls to move with a constant velocity, or oscillate in time.
The [fix wall/region](fix_wall_region) command offers the most
generality, since the region surface is treated as a wall, and the
geometry of the region can be a simple primitive volume (e.g. a sphere,
or cube, or plane), or a complex volume made from the union and
intersection of primitive volumes. [Regions](region) can also specify a
volume \"interior\" or \"exterior\" to the specified primitive shape or
*union* or *intersection*. [Regions](region) can also be \"dynamic\"
meaning they move with constant velocity, oscillate, or rotate.

The only frictional idealized walls currently in LAMMPS are flat or
curved surfaces specified by the [fix wall/gran](fix_wall_gran) command.
At some point we plan to allow region surfaces to be used as frictional
walls, as well as triangulated surfaces.