Your system can be periodic in x,y and apply NPT in those dims,
and non-periodic in z, with a wall.
But you can't adjust the volume (via NPT) of a non-periodic dimension.
with a free surface. It doesn't make conceptual sense.
If you have a wall in a non-periodic dim, you can move the wall
to adjust the pressure, but that isn't what NPT does. Other fixes
do that.
Steve