1. Contribution from the Department of Physics, Monash University, Clayton, Victoria 3168, Australia, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, England, School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia, Department of Physics, Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, England, and Neutron Science Division, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, England