1. Department of Chemical Physics, Budapest University of Technology and Economics, Budapest H-1521, Hungary, Department of Applied Analysis, Institute of Mathematics, Eötvös University, P.O. Box 120, Budapest H-1518, Hungary, Department of Applied Mathematics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, and Department of Physical Chemistry, Institute of Chemistry, Eötvös University, P.O. Box 32, Budapest H-1518, Hungary