Molecular Solvent Points from a Grid

  I need to select the points in a grid that belong outside
  the van der Waal surface. Although this seems trivial, you
  can think of complications when you look at details. For
  example, checking for points outside the van der Waal Radii
  of each atom seems one "correct" approach, but... In a molecule
  like benzene, you may select points within the ring itself if
  you consider the limiting case of very small van der Waal Radii
  for the carbon atoms.
  Is there any algorithm or program to do this and avoid the pitfall
  described above (and any other I have not  thought of!)?
  Thanks! I will summarize if there is interest!
  Gustavo A. Mercier,Jr.,MD,PhD         (. . `) -._    )-;-,_()
  Division of Nuclear Medicine          (v_,)'  _  )`-.\  ``-
  Dept. of Radiology                    _;- _,-_/ / ((,'
  University of Pennsylvania           ((,.-'  ((,/
  3400 Spruce St.                  gmercier \\at//
  Philadelphia, PA 19104         215-662-3069/3091 fax: 215-349-5843