1. Dipartimento di Scienze Chimiche, Università di Cagliari-CSGI, Cittadella Monserrato, S.S. 554 Bivio Sestu, 09042 Monserrato-Cagliari, Italy, Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040, Germany, and Department of Applied Mathematics, The Australian National University, Canberra, Australia