1. Laboratory for Computational Biology, Shandong Provincial Research Center for Bioinformatic Engineering and Technique, Shandong University of Technology, Zibo 255049, P. R. China, Institute of Theoretical Chemistry, Shandong University, Jinan 250100, P. R. China, and Department of Chemistry, Shandong Teachers' University, Jinan 250014, P. R. China zhang630@mail.zbu.net.cn