1. Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education; Tsinghua University; Beijing 100084 P.R. China
2. Physical and Theoretical Chemistry; University of Siegen; Siegen 57068 Germany
3. Theoretical Division; Los Alamos National Laboratory; Los Alamos New Mexico 87545 USA
4. Environmental Molecular Science Laboratory; Pacific Northwest National Laboratory; Richland Washington 953002 USA