1. Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 3QZ, U.K., Department of Chemistry and Center for Molecular Modeling, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, The Institute of Mathematical Sciences, C.I.T Campus, Taramani, Chennai 600 113, India, and Department of Biochemistry and Molecular Biophysics, University of Pennsylvania, 37th and Hamilton Walk, Philadelphia, Pennsylvania 19104-6059