Pressures for a fixed free surface in a vaccuum


I am doing some different models where I equilibrate a structure in fully periodic npt (temps and pressures balance nicely).

When I open up the surface for this equilibrated structure (moving boundaries to p p f and creating a vaccuum in the z) the pressure now goes up a fair amount. I run an nvt or a langevin but of course this does not cause the pressure to go down.

In the past I have not worried too much about this (see response to Relaxation for no-period boundary condition box), but is there a reason why the pressure goes up (or down) when a free surface is created?

As a follow-up/clarifiaction, I suppos I could artificially drop the pressure I am seeing.

What my question moreso is is whether this pressure I see when opening the surface (from a previously well equilibrated periodic structure) is:

  • a product of what happens when a surface is introduced
  • irrelevant because pressure is not well defined during free surfaces/vaccuums
  • a product of an error in my simulation approach (the thing I am trying to avoid).