1. Max-Planck-Institut für Physik komplexer Systeme, Nöthnitzer Strasse 38, D-01187 Dresden, Germany, Department of Chemistry, Rice University, Houston, Texas 77005, Department of Chemistry, Faculty of Natural Sciences, Imperial College London, SW7 2AZ, London, U.K., and Institut für Festkörperforschung, Theorie-II, Forschungszentrum Jülich, D-52425 Jülich, Germany