1. Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA
2. Department of Chemistry, Norwegian University of Science and Technology, 7491 Trondheim, Norway
3. Quantum Theory Project, Departments of Chemistry and Physics, University of Florida, Gainesville, Florida 32611, USA
4. Scuola Normale Superiore, Piazza dei Cavaleri 7, 56126 Pisa, Italy