Comparison between CASPT2 and DFT in the Study of Ni(C₂H₄)₂ Complexes

In this paper we report the results of a comparative theoretical study at the CASSCF/CASPT2 and DFT levels of the potential energy surface associated with bis(ethylene)−nickel complexes. The computations predict the existence on the potential surface of only one minimum of D 2 d symmetry: the other critical points located on the surface (corresponding to structures of C 2 v and D 2 h symmetries) are saddle points of higher order. The results have pointed out that (i) the inclusion of dynamic correlation does not change the topology of the surface; (ii) the DFT approach provides for these systems results very similar to those obtained at the CASSCF/CASPT2 level; (iii) an effective core potential (ECP) basis (LANL2DZ) can provide a reliable description for these metal−olefin clusters. The second and third points are of particular interest since they indicate that it is possible to use a cheap approach (DFT with an ECP basis) to investigate larger nickel−olefin clusters with bulky ligands on the metal atom, as those involved in catalyzed cycloaddition reactions.