F.Bernardi A.Bottoni M.Calcinari I.Rossi M.A.Robb Comparison between CASPT2 and DFT in the study of Ni(C2H4)(2) complexes JOURNAL OF PHYSICAL CHEMISTRY A 101 6310–6314 1997 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-2d symmetry: the other critical points located on the surface (corresponding to structures of C-2v and D-2d 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.