1. Contribution from the Dipartimento di Chimica Fisica “F. Accascina”, Viale delle Scienze - Parco d'Orleans II, I-90128 Palermo, Italy, Department of Chemistry, Emory University, Atlanta, Georgia 30322, and Department of Theoretical Chemistry, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden