</H3>
<PRE>
invoke 2-d slab Ewald/PPPM and set extended slab volume via this ratio
2-d slab Ewald/PPPM can be used for a system that is periodic in x-y,
  but not in z
this ratio dampens inter-slab interactions in the z dimension 
  by providing empty volume between slabs and removing
  dipole inter-slab interactions
ratio value is the size of the extended dimension in z divided by
  the actual dimension in z
recommended ratio value is 3.0: larger is inefficient, smaller
  risks unwanted inter-slab interact