1. Contribution from the Institute of Physics, Stockholm University, P.O. Box 6730, S-11385 Stockholm, Sweden, Laboratoire de Physique Quantique, Université Paul Sabatier, F-31062 Toulouse, France, Theoretical Chemistry, Department of Biotechnology, Royal Institute of Technology (KTH), S-10044 Stockholm, Sweden, and Inorganic Chemistry, Department of Chemistry, Royal Institute of Technology (KTH), S-10044 Stockholm, Sweden