Despite the good performance of the SCAN functional in predicting the relative energies of the pure MnO 2 polymorphs (Fig. ), in the absence of a correction scheme, both SCAN and PBE functionals display considerable errors in predicting Mn oxide formation energies (Fig. ). This finding is consistent with previous investigations of transition metal oxides. While DFT errors tend to cancel when calculating the relative energy of two compounds with similar electronic structures, errors persist in formation energy calculations (Eqs. 1-5), which compare compounds with different oxidation states. Implementing the correction scheme detailed in the methods section improves the accuracy of the computed formation energies by a factor of 5 (Fig. , averaged across the three functionals). However, mean absolute error of over 100 meV/atom remains (Fig. at U = 0), frustrating reliable investigation of redox energetics. To further counteract self-interaction errors, we fit an effective Hubbard-U value to the calibration set of Mn oxides (Fig. ). Unfortunately, no single U eff can exactly reproduce the energies of all Mn oxides simultaneously. The computed energies of small bandgap semiconductors with high covalency (small Mn 3d -O 2p charge transfer gaps) like b-MnO 2 increase with increasing U eff , while materials with greater ionic-character like Mn(OH) 2 exhibit decreasing energy with increasing U eff . For both PBE and SCAN, the Hubbard-U value that minimizes errors across the entire calibration set is between 2.5 eV and 2.75 eV (Fig. ). Since the error is equivalent at these two values, we chose to perform all subsequent calculations with U eff = 2.75eV, corresponding to mean absolute errors (MAE) of 12.2 meV/atom for PBE and 18.2 meV/atom for SCAN.