1. Institute of Theoretical Physics, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225 Düsseldorf, Germany
2. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
3. Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria
4. Center for Computational Material Science and Institute of Theoretical Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, A-1040 Vienna, Austria