Switching off the non-bonded interactions

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Dear LAMMPS users and developers,

In order to switch off the interactions between two groups (one atom type in a solid slab and liquid molecules placed atop the surface), I’m using a hybrid pair_style, with one being “lj/charmm/coul/long” (among solid-solid and liq.-liq. particles) and the other “lj/cut/coul/long/soft” (one solid atom type-liquid). Regardless of the value of the coupling parameter used for the latter style, the surface is very unstable, such that the top-layer atoms tend to move into the liquid phase. I don’t run into this problem when using only the “lj/charmm/coul/long” command (which is the full-interaction case). Could you please let me what I’m doing wrong here? I assumed the easier alternative approach is using the “neigh_modify exclude” command, however that didn’t work either.
I’d appreciate your comments.

Best regards,
Monir

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if you turn off the interactions between the two phases of your system, then those atoms won’t “see” each other. thus you turn your simulation into a superposition of each phase being next to a vacuum. now it depends on whether you have a system with a stable surface in each part. if not, those atoms can move into the space where the other phase is, because they don’t interact anymore and thus are not repelled by them.

thus for the simulation to work as expected, you have to first test each part independently to give the expected behavior. if not, there is either a problem in that you start from an unstable surface or that there is a problem with your force field or your simulation settings or a combination of those.

axel.