1. IBM Thomas J. Watson Research Center, Route 134 and PO Box 218, Yorktown Heights, New York 10598, Department of Chemistry and Center for Biomolecular Simulation, Columbia University, New York, New York 10027, Department of Chemistry, Yale University, New Haven, Connecticut 06520, and Department of Chemistry, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854