1. Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2, I-40126 Bologna, Italy, Equipe de Chimie et Biochimie Théoriques, UMR 7565 - CNRS, Université Henri Poincaré, Nancy 1, BP 239, 54506 Vandoeuvre-lès-Nancy Cedex, France, and Department of Chemistry, Imperial College London, London SW7 2AZ U.K.