To calculate formation enthalpies, all compounds were referenced to their constituent elements at standard state. A correction scheme was applied to rectify well-known DFT errors: namely, overbinding of O 2 and incomplete cancelation of self-interaction errors. To address the former, our correction scheme adjusts the energy of O 2 to reproduce the experimental formation enthalpy of water, consistent with previously reported schemes. To minimize error arising from incomplete self-interaction cancelation, the energy of Mn metal was set to the average value calculated from five experimental formation energy reactions (eq. 1-5, Table ). Critically, the five reactions include Mn ions with +2, +3, and +4 oxidation states. Averaging the Mn metal reference energy across these reactions is likely to achieve a reasonable balance in the accuracy of DFT-computed formation energies across all oxidation states of AxMnO2 • yH2O computed in this work.