1. Contribution from the Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1, Istituto di Chimica Farmaceutica, Università di Milano, I-20131 Milano, Italy, Chemistry Department, Brookhaven National Laboratory, P.O. Box 5000, Upton, New York 11973, Institut Max von Laue - Paul Langevin, 38042 Grenoble Cedex, France, and...