1. Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K., Center for Molecular Modelling and Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pensylvannia 19104-6323, and Department of Chemistry and Applied Biosciences, ETH Zurich, USI - Campus, via Giuseppe Buffi 13, 6900 Lugano, Switzerland