1. Department of Chemistry; North Carolina State University; Raleigh NC 27695-8204 USA
2. Key Laboratory of Computational Physical Sciences (Ministry of Education); State Key Laboratory of Surface Physics; Department of Physics; Fudan University; Shanghai 200433 P. R. China
3. Collaborative Innovation Center of Advanced Microstructures; Nanjing 210093 P. R. China
4. Max-Planck-Institut für Festkörperforschung; Heisenbergstrasse 1 70569 Stuttgart Germany