1. Department of Chemistry, H. C. Ørsted Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark, Departament de Química Física, Universitat de Barcelona, Martí i Franqués 1, 08028 Barcelona, Spain, and Department of Mathematics and Statistics, University of Surrey, Guildford, GU2 5XH, U.K.