1. Center for Molecular Modeling and Department of Chemistry, University of Pennsylvania, 231 S. 34th Street, Philadelphia, Pennsylvania 19104-6323, Department of Chemistry, Division of Molecular and Life Science, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea, Centro Svizzero di Calcolo Scientifico (CSCS), via Cantonale, 6928 Manno, Switzerland, Physical Chemistry, ETH Zurich, Hönggerberg, 8093 Zurich, Switzerland