Can single reference density functional theory methods describe spin state crossing for 3d transition metal carbon...

The bonding between a transition metal (TM) and carbon monoxide is found in many chemical complexes that are important in biological and catalytic processes. Density functional theory (DFT) methods have been widely used for many calculations; however, the validity of such calculations for complex spin state coupling induced by TM-CO bond formation remains unclear. Interestingly, binding between a 3d TM atom and carbon monoxide induces a spin state change, making it a good benchmark system for evaluating various functionals. There have been numerous studies evaluating the TM atom spin state splitting or spin state energy difference of CO-bound TM complexes. However, we did not find benchmark studies evaluating the potential energy surfaces of the TM-CO association for various spin states to determine the spin-state crossing point. In the present study, we calculated the 3d TM + CO association potential energy curve using various DFT functionals. For TM = Sc, Ti, Fe, Co, and Ni, we performed multireference calculations with multiple excited states to clarify the spin state crossing points. We found that hybrid functionals can give spin state crossing points within 0.15 Å of those obtained by multireference methods, and confirmed that accurate atomic spin splitting results in more accurate crossing point geometry. In addition, we found minimal basis set dependence for the association potential energy curve for B3LYP. To our surprise, this study showed that hybrid DFT functionals can describe spin crossing phenomena in the TM + CO association.