In the Ru[NH 3 ] 3+ 6 -Au system, the electrode potential differences inferred from inner potential and Fermi levels are small and within 0.1 V. The difference corresponds to ∼ 5% differences in the applied electrode potential. For quantitatively accurate calculations this is already a notable difference and a 5% over-potential (η) error leads to a factor of 7 and 3 error in the computed reaction thermodynamics and rates, respectively. 1 We expect that larger differences between the inner potential and Fermi level methods can be found for electrodes with a low DOS around the redox potential such as graphene, densely charged anionic outer-sphere species such as O - 2 , or when a very small slab is used to present the electrode. The electrode potential differences inferred from either Fermi level and inner potential methods are also expected to become more pronounced when the surface PZC and the redox potential of the outer-sphere couple differ significantly.