1. Department of Chemistry, University College London, 20 Gordon Street, London, UK, WC1H OAJ, COSMOlogic GmbH and Co. KG, Burscheider Str. 515, Leverkusen 51381, Germany, and Statistical Sciences, GlaxoSmithKline, NFSP, Third Ave, Harlow, Essex, CM19 5AN, UK