Supporting Information Reduction enthalpy and charge distribution of substituted ferrites and doped ceria for thermochemical water and carbon dioxide splitting with DFT+U Dimitrios A. Dimitrakis, Nikolaos I. Tsongidis, Athanasios G. Konstandopoulos S1. Calculation of the chemical potential of oxygen The chemical potential of an Oxygen atom, μO, is taken to be half of the calculated total energy of a free, isolated, spin polarized oxygen molecule (triplet) with the GGA-PBE method, using a supercell of 12x12x12Å3, a cutoff energy of 520eV, forces criterion of under 0.001eV/Å, electronic ground state convergence of 10-7eV and the PAW PBE O potential supplied with VASP version 4.6. Because GGA is known to significantly overestimate the binding energy of O2even by more than 1.5eV [1-3], a correction was applied to the O2total energy, by comparing the calculated binding energy to the experimental value of 5.12eV [4,5]. We calculate a binding energy of 6.65eV, which is higher than the experimental value by +1.53eV. This correction is applied to the oxygen molecule total energy bringing it higher to –8.32eV, which lies close to previous GGA calculations . A zero point energy correction has not been considered in the present study. The correction follows equations (S1)-(S3): BEcalc= 2EOat− EO2mol(S1)Ecorr= BEcalc− BEexp(S2)EO2corr= EO2mol+ Ecorr(S3)where, BEcalcis the calculated binding energy of the oxygen molecule, EOatis the total energy of a single oxygen atom, EO2molis the total energy of an oxygen molecule, Ecorris the correction applied, defined as the difference between the calculated binding energy and the experimental binding energy BEexp, and EO2corris the corrected oxygen molecule total energy used in the calculations. S2. Hubbard term (U) The approach of Dudarev et al.  is used in this study, with a single effective interaction parameter, U-J, to define the on-site Coulomb interaction, in which the U parameter and the exchange parameter J are combined under a spherically average, named simply as U.