1. Department
of Physics, University of Oulu, P.O. Box 3000, FI-90014 University of Oulu, Finland
2. Division of Theoretical Chemistry & Biology, School of Biotechnology, Royal Institute of Technology, S-106 91 Stockholm, Sweden
3. Department
of Physics, University of Helsinki, P.O. Box 64, FI-00014 University of Helsinki, Finland
4. Swedish
e-Science Research Centre (SeRC), KTH Royal Institute of Technology, S-100 44 Stockholm, Sweden