1. Department of Physical Sciences, University of Helsinki, P.O. Box 64, FIN-00014 University of Helsinki, Finland, Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 University of Helsinki, Finland, Institute of Environmental Physics, University of Tartu, 18 Ülikooli Str., 50090 Tartu, Estonia, and CSCScientific Computing Ltd., P.O. Box 405 (Keilaranta 14), FIN-02101 Espoo, Finland