Can DFT Accurately Predict Spin Densities? Analysis of Discrepancies in Iron Nitrosyl Complexes

Iron nitrosyl complexes are a particularly challenging case for density functional theory. In particular, for the low-spin state, different exchange–correlation functionals yield very different spin densities [ Conradie J. ; Ghosh A. J. Phys. Chem. B 2007, 111, 12621−12624 ]. Here, we investigate the origin of these differences in detail by analyzing the Kohn–Sham molecular orbitals. Furthermore, to decide which exchange–correlation functionals yield the most accurate spin densities, we make comparisons to CASSCF calculations. To ensure that the spin densities are converged with respect to the size of the active space, this comparison is performed for [Fe(NO)]2+ as a model system. We find that none of the investigated exchange–correlation functionals are able to reproduce the CASSCF spin densities accurately.