1. Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040, Germany, Dipartimento di Scienze Chimiche, Università di Cagliari−CSGI, Cittadella Monserrato, S.S. 554 Bivio Sestu, 09042 Monserrato-Cagliari, Italy, and Department of Applied Mathematics, A.N.U. Canberra, Australia, and Visiting Professor, Cagliari, Florence, and Regensburg University