1. Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, SW7 2AY, U.K., School of Applied Chemistry, Curtin University of Technology, P.O. Box U1987, Perth, Western Australia 6845, Molecular Simulations, Inc., The Quorum, Barnwell Road, Cambridge, CB5 8RE, U.K., and Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.