1. Theoretische Physik, Universität Paderborn, Warburger Strasse 100, D-33098 Paderborn, Germany, BCCMS, Universität Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany, Department of Physics, University of Strathclyde, John Anderson Building, 107 Rottenrow, Glasgow G4 0NG, United Kingdom, and Institute for Material Chemistry, Vienna University of Technology, Getreidemarkt 9/165-TC, A-1060, Vienna, Austria