A geometry optimization on a molecular system containing a transition metal center and a flexible ligand will certainly yield a minimum on the potential energy surface (PES). However, there is no guarantee that the optimized structure is a global minimum on the PES, but a local minimum in most cases. The searching for the global minimum of each intermediate and the transition state is of crucial importance for mechanistic understanding on a specific reaction. In this work, three transition states (TS1, TS2, TS3) for the Cr/PCCP catalyzed ethylene tri-/tetramerization were selected as the molecular models, which were subjected to a sophisticated conformational search using Tinker program and CREST program, respectively. With a careful analysis, the global minimum for each of these three transition states was successfully located using either Tinker program or CREST program, and both methods generated a similar number of conformers in this study. Compared with Tinker program, the conformational screening using CREST program is more straightforward and easier to use.

Key words: transition metal complex, flexible ligand, conformational screening, density function theory (DFT)