1. H.H. Wills Physics Laboratory, Tyndall Ave., Bristol BS8 1TL, United Kingdom
2. School of Chemistry, Cantock’s Close, University of Bristol, Bristol BS8 1TS, United Kingdom
3. Centre for Nanoscience and Quantum Information, Tyndall Avenue, Bristol BS8 1FD, United Kingdom
4. Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA
5. Institut für Theoretische Physik II, Heinrich-Heine-Universität, D-40225 Düsseldorf, Germany