[lammps-users] about wall in lammps

you can just find the archive of the maillist. I got the simular help as bellow.

I use a lot of the FORTRAN version of lammps and it did not have this =
fix wall command. I usually needed a wall for my slab simulations and =
what I do was create a sheet of atoms that are periodically arranged (so =
that I can have periodic boundaries along x and y and my walls are =
located at the z-axis) . I define the wall’s interaction potential which =
is usually truncated 6-12 LJ and put a fix to set the force and velocity =
of these atoms to zero (so that they will not move).=20
For your case, you can create a cylinder made up of atoms that are =
hexagonally packed so that it would be impenetrable and set a fix to =
these atoms so that the cylinder will not move. And, put your liquid =
crystals inside the cylinder.=20
By the way, I would define all the atoms coordinates at the data file.