Total number of atoms (particles).  This variable is usually not set manually 
by the user, but is set automatically when commands such as <tt>makecrystal</tt>
and <tt>readcn</tt> is called.  In Tcl the value of this variable can be printed
out by, e.g.

<p>
<tt>
 puts "Number of atoms = [MD++_Get NP]"
</tt>

