1. Department of Chemistry Minnesota Supercomputing Institute and Chemical Theory Center University of Minnesota 207 Pleasant Street SE Minneapolis MN 55455-0431 USA
2. Current address: Key Laboratory of Cluster Science of Ministry of Education School of Chemistry and Engineering Beijing Institute of Technology Beijing 100031 P. R. China
3. Department of Civil & Environmental Engineering & Earth Sciences University of Notre Dame 301 Stinson-Remick Hall Notre Dame IN 46556 USA