where C sol is the solution capacitance and C M is the metal capacitance. We assume C sol as the capacitance with z im -z a = 0 from the semi-classical simulations, i.e. 5.95 and 6.09 µF/cm 2 for Au(111) and Au(100) respectively. These baseline values are comparable with those obtained in previous studies with similar types of models. Therefore, Eq. 5 provides a theoretical prediction on the effect of image plane on the total capacitance, which can be checked with simulations.