1. Schlumberger Cambridge Research, Madingley Road, High Cross, Cambridge, CB3 0EL U.K., Element Six B.V., Beversestraat 20, 5431 SH Cuijk, The Netherlands, Element Six Ltd., King's Ride Park, Ascot, Berkshire, SL5 8BP U.K., and Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ U.K.