ions
when 2-d slab Ewald/PPPM is used, z-direction periodicity must be
  turned off - e.g. periodicity 0 0 1
when 2-d slab Ewald/PPPM is used, user must prevent particle migration
  beyond initial z-bounds, typically by providing walls
2-d slab Ewald/PPPM can only be used only with electrostatically
  neutral systems
2-d slab Ewald/PPPM can only be used (for the moment) with constant
  volume simulations (no pressure control) - the pressure computation
  (printed as thermodynamic data) does not include any 