[Zaphod_B]

The (Gibbs) energy change of water oxidation, 2 H₂O  O₂ + 2 H₂, should be 4x1.23 eV. However, using DFT (B3LYP/6-31G*), this number comes up short. My results match those on CCCBDB, around 4.04 eV. Why is this?

I've seen a lot of papers simply set the gas phase hydrogen and oxygen to 4x1.23 eV and reference Atkins Phys. Chem., and some others add energy of solvation to the catalyst to achieve the minimum energy.