Yes! I could resolve the problem. J
In my case the solid carbon structure (adsorbent) was rigid and immobile.
I found out the reason for high potential energy was contribution of solid-solid LJ interactions! Considering solid atoms are bounded by covalent bonds, they are located within a distance smaller than their solid-solid sigma. As such, they generate a huge positive contribution into the total potential energy. Therefore, I simply switched those interactions off, so that LAMMPS now only calculates solid-fluid and fluid-fluid interactions and everything is fine.
Hope it helps.
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