1. Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany, Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63108, Department of Chemistry and Center for Biomolecular Simulations, Columbia University, New York, New York 10027, and Department of Organic Chemistry and the Lise-Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel