Questions about this have come up on
the mail list previously, so FYI, the 9 Feb
patch introduces a new "change_box" command
which replaces the old "displace_box" and
"change_box" commands. The new one
allows you to change the size/shape of the
box as well as the boundary conditions,
e.g. from periodic to non-periodic.
It also gives you a good tool for losing
atoms, overlapping atoms, and in general
totally screwing up your simulation, so enjoy.
Steve