1. Contribution from the Institut für Physikalische und Theoretische Chemie, J. W. Goethe-Universität Frankfurt, Marie-Curie-Str. 11, D-60439 Frankfurt/Main, Germany, School of Computational Sciences, Korea Institute for Advanced Study (KIAS), 207-43 Cheongnyangni 2-dong, Dongdaemun-gu, Seoul 130-722, Korea, and National Creative Research Initiative Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Pohang 790-784, Korea