1. Contribution from the Dipartimento di Chimica, Università della Basilicata, Via N. Sauro, 85, 85100 Potenza, Italy, Afdeling Theoretische Chemie, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, and Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands