1. Division of Quantum and Physical Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium, Department of Theoretical Chemistry, Chemical Center, Lund University, P.O. Box 124, 221 00 Lund, Sweden, and Departamento de Química, C-14, and Instituto Universitario de Ciencia de Materiales Nicolás Cabrera, Universidad Autónoma de Madrid, 28049 Madrid, Spain