Quantifying the effects of the self-interaction error in density functional theory: when do the delocalized states appear? II. Iron-oxo complexes and closed-shell substrate molecules.

Effects of the self-interaction error (SIE) in approximate density functional theory have several times been reported and quantified for the dissociation of charged radicals, charge transfer complexes, polarizabilities, and for transition states of reactions involving main-group molecules. In the present contribution, effects of the SIE in systems composed of a catalytic transition metal complex and a closed-shell substrate molecule are investigated. For this type of system, effects of the SIE have not been reported earlier. It is found that although the best density functionals (e.g., B3LYP) are capable of accurate predictions of structure, thermodynamics, and reactivity of such systems, there are situations and systems for which the magnitude of the SIE can be large, and for which the effects can be severe for the modeling of chemical reactivity. The largest energetic effect reported here is the artificial stabilization of a catalyst-substrate complex by as much as 18 kcal/mol. Also, the disappearance of significant energy barriers for hydrogen atom transfer in certain systems are reported. In line with earlier work, it is found that the magnitude of the SIE is related to the energetics of electron transfer between the metal catalyst and the substrate molecule. It is suggested that these problems might be circumvented by the inclusion of counterions or point charges that would alter the energetics of electron transfer. It is also pointed out that the effects of SIE in the modeling of transition metal reactivity need to be investigated further.