1. Institute of Chemistry, Laboratory of Theoretical Chemistry, ELTE Eötvös Loránd University, P.O. Box 32, H-1518, Budapest 112, Hungary
2. Max-Planck-Institut für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany
3. Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada