We start by investigating the prototype reaction discussed in Figure , i.e. H + + e -+ C * → CH * . Figure shows the variation of the energy and Figure (points) shows q (Equation Figure : a) Raw energies from images used in a nudged-elastic band calculation of C * + H + + e -→ CH * b) (points) (q 2 -q 1 ), where q 2 refers to FS and q 1 to the IS, obtained for all images for the reaction H + + e -+ C * → CH * (lines) erfc fit to show the change in charge along reaction path c) Energy vs. mean potential based on fitting (black line) and the predictions from finite difference for different cells (colors) 17) along the reaction path. We consider just the proton and the adsorbed C atom in the summation of Equation . q goes through a maximum at the TS falling to zero when the CH * species is formed, mirroring the change in energy with reaction path. Different proton concentrations, spanning a potential range of 2 V in potential, are consistent in their force-based prediction of q ≈ 0.3e, which is similar to the value of q = 0.37e from fitting over the varying cell sizes in Figure .