1. Department of Chemistry, University of Leuven, Celestinijnenlaan 200F, B-3001 Leuven, Belgium, Department of Computational Methods in Chemistry, Jagiellonian University, R. Ingardena 3, 30-060 Cracow, Poland, and K. Gumiński Department of Theoretical Chemistry, Jagiellonian University, R. Ingardena 3, 30-060 Cracow, Poland