      (4) desired Py at end of run
               (5) desired Pz at beginning of run
               (6) desired Pz at end of run
               (7) frequency constant for volume adjust (inverse time units)

enable constant pressure simulations
all specified pressures are in pressure units
any dimension being varied by pressure control must be periodic
for xyz coupling, all 3 dimensions expand/contract together uniformly
  using total scalar pressure as the driving force
for xy/yz/xz coupling, the 2 specifie