1. Department of Theoretical Chemistry, Chemical Center, University of Lund, P. O. Box 124 S-221 00 Lund, Sweden, Department of Physical Chemistry, Sciences II, University of Geneva, Quai E. Ansermet 30, 1211 Geneva 4, Switzerland, Instituto de Ciencia Molecular, Universitat de València, P. O. Box 22085, ES-46071 Valencia, Spain, and Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, P. O. Box 1033 Blindern, N-0315 Oslo, Norway