[1] Center for Scientific Computing, P. 0. Box 405, FIN-02101 Esbo, Finland
[2] Department of Physics, University of Jyväskylä, P. O. Box 35, FIN-40351 Jyväskylä, Finland
[3] Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, FIN-02015 HUT, Finland
[4] Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark