1. Institute of Physics, Stockholm University, P.O. Box 6730, S-11385 Stockholm, Sweden, Laboratoire de Physique Quantique, Université Paul Sabatier, F-31062 Toulouse, France, Lehrstuhl für Theoretische Chemie, Technische Universität, Lichtenbergstrasse 4, D-85747 Garching, Germany, and Inorganic Chemistry, Department of Chemistry, Royal Institute of Technology (KTH), S-10044 Stockholm, Sweden