1. Contribution from the Department of Chemistry, University of Calgary, 2500 University Drive, N.W., Calgary, Alberta, Canada T2N 1N4, MD-IM-FA Computational Chemistry, Bayer AG, Gebäude Q18, D-51368 Leverkusen, Germany, and Zurich Research Laboratory, IBM Research Division, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland