eriodic BC in x direction (0) yes, (1) no 
    <LI>
    2nd parameter = periodic BC in y direction (0) yes, (1) no 
    <LI>
    3rd parameter = periodic BC in z direction (0) yes, (1) no 
</UL>
<PRE>
turn on/off periodicity in any of three dimensions
used in inter-particle distance computation and when particles move
  to map (or not map) them back into periodic box
for a 2-d run (see &quot;dimension&quot; command), 3rd parameter must be 
  specified, but doesn't matter if it is 0 or 1
must be set before d