1. Department of Computational Science, National University of Singapore, Kent Ridge, Singapore 119260, Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, and Center for Theoretical & Computational Chemistry and State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, P.R. China 116023