redefine region

Please post to the list, not to me.

The LAMMPS formula for pressure has a volume in
it. That is the box volume.

The LAMMPS formula for per-atom stress does
not have a volume in in it. You can define it
be whatever you want. If you choose the box
volume, then you will get the same answer as
for pressure (if you sum over all atoms). See
the compute stress/atom doc page for details.