[lammps-users] Seeking for help regarding changing the box size


I am trying to build a polymer thin film with one free surface. The x and y direction of the simulation box is periodic and the z direction is of fixed boundary condition. The zlo has a reflecting wall and the zhi has a lj 9-3 wall. I am able to deform the box in the x and y direction using the deform command but it shows an error message if I try to deform the z direction. I have tried to implement the move command in the z direction but it shows an “illegal wall command”.Is there a way around this?

Samyabrata Chatterjee

Conceptually, deforming a material with free surfaces is somewhat illogical.
What is acting on the material to deform it? Free surfaces imply it goes where
it wants to go. That is why fix deform is not allowed in a non-periodic dim.

If you want to apply fictitious forces with fix move, then your atoms have to stay
within the walls. Some of the wall commands have options to move the position
of the wall as a function of time. If you use that feature consistent with fix move
on the mobile atoms, you should be OK.