1. Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom, Department of Chemistry and CNISM, University of Rome La Sapienza, Piazzale A. Moro 5, 00185 Rome, Italy SuperComputing Center CASPUR, via dei Tizii 6, 00185 Rome, Italy, and Laboratoire de Chimie Theorique, Université Henri Poincare, F54506 Vandoeuvre-les-Nancy, France