1. Theoretical Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands, Department of Chemical Engineering, Universiteit van Amsterdam, Nieuwe Achtergracht 166, NL-1018 WV Amsterdam, The Netherlands, and IBM Research Division, Zurich Research Laboratory, Saumerstrasse 4, Rüschlikon, CH 8803, Zurich, Switzerland