1. Materials Research Department, Risø National Laboratory, Technical University of Denmark, DK-4000 Roskilde, Denmark, Center for Atomic-Scale Materials Design, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark, Niels Bohr Institute, University of Copenhagen, DK-1017 Copenhagen, Denmark, Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germany