1. Kenneth S. Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California at Berkeley, Berkeley, California 94720-1460, UPMC Université Paris 06, UMR 7616, Laboratoire de Chimie Théorique, Case courrier 137 4 place Jussieu, 75005, Paris, France, CNRS Centre National de la Recherche Scientifique, UMR 7616, 75252 Paris, France, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720