1. Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond Virginia 23713, United States
2. Department of Chemistry, Chemical Theory Center and the Minnesota Supercomputing Institute, The University of Minnesota, Minneapolis, Minnesota 55455, United States
3. Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States
4. School of Pharmaceutical Sciences and Technology, Tianjin University, Tianjin, 300072 P.R. China