1. Leiden Institute of Chemistry, Leiden University, Leiden 2300 RA, The Netherlands
2. Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113 0033, Japan
3. Institute for Theoretical Chemistry, University of Stuttgart, 70569 Stuttgart, Germany
4. Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands