1. Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, U.K., Institut für Theoretische Physik, Technische Universität Dresden, Mommsenstrasse 13, D-01069 Dresden, Germany, and Dipartmento di Chimica G. Ciamician, Universitá di Bologna, via F. Selmi 2, 40126 Bologna, Italy