1. Department of Chemistry and Biochemistry, University of Bern, CH-3012 Bern, Switzerland, Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain, Department of Chemistry, University of Basel, CH-4056 Basel, Switzerland, Institut für Theoretische Festkörperphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany, and Institute for Nanotechnology, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany