Bridging calculated charges and formal oxidation state of manganese oxides: a DFT/DFT+U study

Bridging the gap between theoretically predicted and measured metal oxides charges is crucial when using synergistically computational and experimental techniques to ensure reliability of the models. Manganese oxide, with its wide range of oxidation states, is an ideal candidate for probing this relationship. Here, we conducted a magnetic moment and Bader charge analysis using two different exchange correlation functionals, i.e. RPBE and BEEF-vdW, both with DFT (U = 0 eV) and DFT+U (U = 2.5 eV), on several bulk and cluster manganese oxides with formal Mn oxidation states ranging between +1 and +7. We found that the relationship between Mn formal oxidation states and magnetic moments in both bulk and molecular structures can be described accurately by a quadratic fit. In comparison, the relationship between formal oxidation states and Bader charge is more uncertain, and could be fit by a single hyperbolic function only upon correction of the formal oxidation state via the Madelung constant. Finally, we employed the derived correlations to predict the formal oxidation state of Mn in different Mn_xO_yH_z clusters on fcc-Co(111). Both methods predict that the Mn oxidation states largely do not align with the stoichiometry of the clusters. While the magnetic moments correlation always yielded Mn oxidation states of +2, the results obtained from the Bader charges were more dependent on the cluster stoichiometry.