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 and create_atoms commands, or read in via the 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 or fix nvt is not used with the group that contains wall particles, their positions and velocities will not be updated.
fix aveforce - set force on particles to average value, so they move together
fix setforce - set force on particles to a value, e.g. 0.0
fix freeze - freeze particles for use as granular walls
fix nve/noforce - advect particles by their velocity, but without force
fix move - prescribe motion of particles by a linear velocity, oscillation, rotation, variable
For rough walls, it may be useful to turn off pairwise interactions between wall particles via the neigh_modify exclude 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. The bonded particles do interact with other mobile particles.
Idealized walls can be specified via several fix commands. Fix wall/gran creates frictional walls for use with granular particles; all the other commands create smooth walls.
fix wall/reflect - reflective flat walls
fix wall/lj93 - flat walls, with Lennard-Jones 9/3 potential
fix wall/lj126 - flat walls, with Lennard-Jones 12/6 potential
fix wall/harmonic - flat walls, with repulsive harmonic spring potential
fix wall/region - use region surface as wall
The lj93, lj126, colloid, and harmonic styles all allow the flat walls to move with a constant velocity, or oscillate in time. The 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 can also specify a volume “interior” or “exterior” to the specified primitive shape or union or intersection. Regions 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 command. At some point we plan to allow regoin surfaces to be used as frictional walls, as well as triangulated surfaces.