In order to evaluate selectivity trends for two corresponding reactions, namely the H 2 O 2 and O 2 products, free energy diagrams are constructed for an ideal catalyst along with all considered CaMO 3 catalysts (Fig. ). The diagrams have been developed on the basis of the adsorption free energies of the relevant intermediates involved in two-electron, and four-electron water oxidation reactions, specifically OH * , O * , and OOH * . The computational hydrogen electrode (CHE) model is utilized, which is based on the fact that the chemical potential of a proton-electron pair is equivalent to that of gas-phase H 2 at an electrode potential of U = 0.0 V relative to the reversible hydrogen electrode (RHE) . The impact of the electrode potential on the free energy of the intermediates is incorporated by making an adjustment to the electron energy, with a value of -eU . Here, e represents the elementary charge, while U denotes the electrode potential on the RHE scale.