Redox energetics via Hubbard-U. Before computing the energies of A x MnO 2 • yH 2 O phases for which experimental data is unavailable, we compared the accuracy of the SCAN and PBE functionals on a smaller set of compounds (Fig. ), hereafter referred to as the calibration set. The compounds chosen for the calibration set meet two criteria: i) they are predicted to be equilibrium (or very close to equilibrium) phases within water's voltage stability window, and ii) experimental thermochemical data exists for benchmarking (Table ).