Figure shows the cumulative average of the cell potential ∆ϕ (Eq. 1) over 35 ps of DFTMD under applied field D = 0.018 au (σ m = 8.2 µC/cm 2 ). They are averages of 3 independent trajectories for Au(100)/NaCl(aq) as well as for Au(111)/NaCl(aq). The resulting capacitance value for Au(100) is higher (66 µF/cm 2 ) than the value for Au(111) (52 µF/cm 2 ), however, the standard deviations of the potential are overlapping in these two cases which makes them indistinguishable in practice. These trajectories are free from any specific ion adsorption (see Figure in the Supporting Information for the distance of the nearest counter-ions to the polarized surface). To put these numbers into perspective, the Helmholtz capacitance of the Au(100) single crystal in the absence of specific anion adsorption and at comparable surface charge density is about 65 µF/cm 2 from impedance spectroscopy and that of the Au(111) single crystal is about 72 µF/cm 2 from cyclic voltammetry with the Parsons-Zobel analysis. Therefore, the agreement between theoretical and experimental values is excellent for Au electrodes without specific ion adsorption by considering the uncertainties involved both simulations and measurements. The orientation of surface water molecules is known to shift in response to the surface charge of metal electrodes. Although the vibrational density of states of water bilayer look quite similar between Au(111) and Au(100), the response in a single water molecule to electric field was weaker for the surface with higher Au-O adsorption energy. However, how surface water molecules reorient themselves at electrified Au surfaces immersed in electrolyte solutions is less clear and conclusive. In Figure in the Supporting Information). Therefore, our results confirm previous studies of water orientation at PZC for Au(111) but show a clear shift in the peak positions for electrified surfaces in addition to the pronounced intensity. Moreover, we find that water at Au(100) is more responsive to the negative bias while water at Au(111) is more responsive to the positive bias.