1. Theoretical Chemistry Laboratory, Unit of Theoretical and Structural Physical Chemistry, Namur Institute of Structured Matter, University of Namur, Rue de Bruxelles, 61, B-5000 Namur, Belgium
2. University of Bordeaux, Institut des Sciences Moléculaires, UMR 5255 CNRS, Cours de la Libération 351, F-33405 Talence Cedex, France