1. Department of Chemistry, University of Helsinki, P.O. Box 55 (A. I. Virtasen aukio 1), FI-00014 Helsinki, Finland
2. Division of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
3. Center for Scientific Computing, J. W. Goethe-Universität Frankfurt, Max-von-Laue-Strasse 1, D-60438 Frankfurt am Main, Germany