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Re: [lammps-users] Alternative for vacuum boundary conditions
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Re: [lammps-users] Alternative for vacuum boundary conditions

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Fri, 15 Sep 2017 15:07:15 -0400

On Fri, Sep 15, 2017 at 2:29 PM, José Cobeña <jose.cobena.84@...24...> wrote:
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.



Thanks a lot.

José Cobeña.

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Dr. Axel Kohlmeyer  akohlmey@...12...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.