1. Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
2. School of Physics and Astronomy and the Thomas Young Centre for Theory and Simulation of Materials at Queen Mary, University of London, London E1 4NS, U.K.
3. Atomic Weapons Establishment, Aldermaston, Reading, Berkshire RG7 4PR, U.K.