In this work, we have implemented DFTMD simulations for modelling electrified interfaces of Au/NaCl(aq) under finite electric displacement D, which allows us to directly compute the Helmholtz capacitance for both Au(100) and Au(111) surfaces with ionic and electronic degrees of freedom treated on an equal footing. The Helmholtz capacitance that we obtained is about 60 µF/cm 2 , in excellent agreement with (recent) measurements for single crystal Au electrodes with non-specific ion adsorption. The high value of this capacitance we found in our simulations can be understood by studying the electronic response of the Au electrodes and verified by tuning the position of image plane in the semi-classical models for exactly the same chemical composition of the system and electric boundary condition. Unexpectedly, we observed fast Cl adsorption on both Au(111) at PZC and Au(100) with positive free charge within the time scale of DFTMD simulations. This leads to a charge inversion on the anodic polarized Au(100) which produces almost identical water orientational distribution as that found on cathodic polarized Au(100) without Cl adsorption. These observations suggest a non-monotonic charging behavior of Au(100) with positive bias.