1. Faculty of Computing, University of Latvia, 19 Raina Blvd., Riga LV-1586, Latvia, Faculty of Physics and Mathematics, University of Latvia, 8 Zellu Str., Riga LV-1002, Latvia, and Institute for Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga LV-1063, Latvia
2. Department of Theoretical Chemistry, University of Duisburg-Essen, Universitätstr. 2, D-45141 Essen, Germany