Hi
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?