I just re-read my last post and I think it was confusing. To clarify,
here is the short version of my last email:
I --think-- you are trying to freeze (imobilize) a group of atoms in a
simulation at constant pressure, but you are worried that using
"setforce" will screw up the pressure.
Am I correct?
If so, then I recommend that you look at "fix rigid" instead. I was
told that "fix rigid" should not disturb the pressure (but I was too
lazy to verify it).
I hope this helps.
If you try this and it works, please let us know.