1. Contribution from the Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, U.K., Wilhelm Schickard Institut für Informatik und ZBIT Zentrum für Bioinformatik, Universität Tübingen, Sand 14, D-72072 Tübingen, Germany, and Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K