In this section, we review how to obtain the charge transfer coefficient and the potential dependent energies of an elementary reaction, based on the framework of extrapolating to the infinite cell size. As in previous work, we assume that the electrochemical interface is represented by a capacitor of constant capacitance, C and is invariant with respect to the electrode potential and the reaction geometry. We expect this assumption to hold for small changes in potential far from the potential of zero charge (pzc) where water structures are relatively ordered. In such a purely electrostatic view of the electrochemical interface, a redox reaction, such as protonation, would be equivalent to altering the charge stored in this capacitor.