1. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
2. Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
3. Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, 10691 Stockholm, Sweden
4. NASA Ames Research Center, Moffett Field, California 94035-1000, USA