The slopes of the lines in Figure (a,b) correspond to differences in charges stored in the capacitor, q TS→FS = 0.37e and q IS→FS = 0.74e. Physically, q TS→FS represents the charge transferred from the water layer to the metal going from TS to FS. Thus, Equation 4 allows for a clear determination of the charge transfer coefficient from DFT energies and workfunctions. Note that q IS→FS ̸ = 1, which is in line with previous findings. This lower than expected value of the charge transfer coefficient is likely caused by hybridisation of the metal with the proton in the IS. To summarise, there are three critical quantities to obtain potential dependent energies from DFT calculations, as seen from Equation . Two of these quantities, the reaction energy ∆E 1→2 and the mean workfunction (ϕ 1 + ϕ 2 ) /2 are outputs of DFT calculations performed in a single cell size. The charge transfer coefficient, (q 2 -q 1 ), however, requires DFT calculations in several different proton concentrations / cell sizes or the use of charge partitioning schemes.