1. Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA, The Netherlands, Institut für Theoretische Physik A, RWTH Aachen, 52056 Aachen, Germany and Institut für Festkörper-Forschung - Theorie 3, Forschungszentrum Jülich, D-52425 Jülich, Germany, and Nano-Science Center (Department of Chemistry and Niels Bohr Institute), University of Copenhagen, Universitetsparken 5, DK-2100, Copenhagen, Denmark