The cut-off radius of the Verlet neighbor list <tt>RLIST</tt> minus the cut-off radius of the
potential. <tt>SKIN</tt> is usually 0.1 times the cut-off radius of the potential.
This variable is usually not set manually, but is set automatically by the command that 
reads and initializes the potential.

<p>
Also see <tt>refreshnnlist</tt>, <tt>RLIST</tt>.
