1. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, Department of Computer Science, Rensselaer Polytechnic Institute, Troy, New York 12180, Q-Chem, Inc., 5001 Baum Blvd., Suite 690, Pittsburgh, Pennsylvania 15213, and Departamento de Química Teórica, Facultad de Química, Universidad Nacional Autónoma de México, México D. F. 04510