A comparison of density functional theory withab initio approaches for systems involving first transition row metals

Abstract

Density functional theory (DFT), using the B3LYP hybrid functional, is found to give a better description of the geometries and vibrational frequencies of FeL and FeL+ systems that second-order Moller Plesset perturbation theory (MP2). Namely, DFT correctly predicts the shift in the CO vibrational frequency between free CO and the5Σ− state of FeCO and yields a good result for the Fe-C distance in the quartet states of FeCH 4 + . These are properties where the MP2 results are unsatisfactory. Thus DFT appears to be an excellent approach for optimizing the geometries and computing the zero-point energies of systems containing first transition row atoms. Because the DFT approach is biased in favor of the 3d 7 occupation, whereas the more traditional approaches are biased in favor of the 3d 6 occupation, differences are found in the relative ordering of states. It is shown that if the dissociation energy is computed relative to the most appropriate atomic asymptote and corrected to the ground state asymptote using the experimental separations, the DFT results are in good agreement with high levels of theory. The energetics at the DFT level are much superior to MP2 and in most cases they are in good agreement with high levels of theory.