1. Laboratory of Theoretical Physics, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium, European Patent Office, Bayerstrasse 34, D-80335 Munich, Germany, Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, B-9000 Gent, Belgium, and Eenheid Algemene Chemie (ALGC), Free University of Brussels, Pleinlaan 2, B-1050 Brussels, Belgium