1. Theoretical Chemistry, Chemical Center, POB 124, S-221 00 Lund, Sweden, Institut de Carnot Bourgogne, UMR 5209 CNRS, Universite de Bourgogne, Faculte de Sciences Mirande, 9 Av. Alain Savary, B.P. 47870, 21078 Dijon Cedex, France, and PMMH, ESPCI, 10 rue Vauquelin, F-75231 Paris Cedex 05, France