[gmx-users] How to make a soft repulsive potential wall

[Bert]

Hi gmx-users,

I was simulating a water-surface system recently. The water molecules will
escape from the solid surface, evaporate into the vacuum space above the
surface, probably never come back and be trapped into the underpart of the
surface for periodicity. I want to prevent the evaporating water molecules
to be trapped by the lower part of the surface, and my thought is putting a
soft repulsive potential wall just below the z_box. For clarity, the
schematic is showed as follows,
——————
|  vacuum    |
|                 |
|×× wall  ×× | (a soft repulsive potential wall )
|                 |
|                 |
|                 |
|  vacuum    |
|.................|
|....water.....|
|.................|
|-----------------|
|-----solid-----|
——————
My questions are:

1) Note that wall function was is available in gmx4 now, I wonder which
wall_type I should choose. I do not understand the phrase 'LJ integrated
over the volume behind the wall' and 'LJ integrated over the wall surface,
could you give me a little more explanation?

2) I test the both wall_types and each of them works fine (both walls are
uncharged). I find that the LJ interation between the wall and the solid
surface would not be cut-offed when the distance between them is beyond the
vdw cutoff. I wonder if it is feasible that the water molecules will feel
the repulsive force from the wall only within the cufoff distance around the
wall. It would somewhat the same as the distance restraint, but the latter
is hard to implement in intermolecular case.

3) If wall_type = table is used, could I set the dispersion columns to zero?
Does rvdw work in this case?

If you have any suggestions, that would be much appreciated. Thanks in
advance.
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