ox boundaries are in distance units
each dimension is controlled separately
dimensions not specified by a &quot;volume control&quot; command can be left
  alone (constant volume or nonperiodic) or controlled by
  a &quot;press control&quot; command
any dimension being varied by volume control must be periodic
the lo/hi values are the desired global simulation box boundaries at
  the end of the simulation run
at each timestep, the box is expanded/contracted uniformly from its initial
  lo/hi values to the sp