Hi all,

I know that LAMMPS currently does not support vacuum boundary conditions. However, there is the slab option for kspace_modify, but that one is useful when the empty space is desired in the z-direction. Has anyone know any alternative if one wants to use PBC only in z-direction, and the empty space in x and y direction?

Thanks a lot.

Hi all,

I know that LAMMPS currently does not support vacuum boundary conditions.

However, there is the slab option for kspace_modify, but that one is useful

when the empty space is desired in the z-direction. Has anyone know any

alternative if one wants to use PBC only in z-direction, and the empty

space in x and y direction?

you can have any combination of periodic or non-periodic boundary

conditions for the x-, y-, and z- direction. the "kspace_modify slab"

option only applies for cases using a long-range solver. it works by

computing a 3-d periodic system and then solving the poisson equation in

z-direction and use that result to (mostly) remove the impact of PBC in

z-direction.

if your system is only periodic in z-direction *and* requires explicit

coulomb interactions, it may be just as well. to simply use regular

periodic boundaries in z-direction and coul/cut with a long cutoff. unlike

with a 3-d system, the increase of the number of neighbors is not quite as

drastic (due to the finite size in x and y), so you can afford a much

longer cutoff with reasonable impact to performance and also the spherical

cutoff related artifacts will be much reduced in a 1-d periodic system

compared to a 2-d periodic system.

that said, you can also use the MSM method for this purpose. unlike the

fourier space based PPPM and Ewald summation, MSM operates in real space

and can thus be applied to any combination of periodic and non-periodic

boundary conditions. the challenge with MSM is the increased computational

cost for achieving a good accuracy of the forces.

axel.