1. Departament de Quimica–Fisica, Universitat de Barcelona, Avinguda Diagonal 647, 08028 Barcelona, Spain
2. Renewable Resources Assessment Group, Centre for Environmental Technology, Imperial College of Science, Technology and Medicine, London SW7 1NA, UK
3. Department of Mathematics, University of Glasgow, 15 University Gardens, Glasgow G12 8QW, UK
4. Departament d'Estructura i Constituents de la Materia, Universitat de Barcelona, Avinguda Diagonal 647, Barcelona 08028, Spain