1. Chair of Solid-State and Quantum Chemistry , Institute of Inorganic Chemistry, RWTH Aachen University , 52056 Aachen , Germany
2. Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education , Tsinghua University , Beijing 100084 , P. R. China
3. Tsinghua University, Theoretical Chemistry Center , Beijing 100084 , P. R. China
4. University at Siegen, Physical and Theoretical Chemistry Group , 57068 Siegen , Germany