1. Center for Molecular Modeling, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium, Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, B-9000 Gent, Belgium, and Department of Physics, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway