1. University of Oslo, Department of Molecular Biosciences, P.O. Box 1041 Blindern, N-0316 Oslo, Norway
2. University of Oslo, Department of Chemistry, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
3. Lund University, Department of Theoretical Chemistry, P.O. Box 124, S-221 00 Lund, Sweden