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