This gap between experimental and theoretical investigation of EDLs at metal-electrolyte interfaces is partly due to the challenges of molecular modelling of electrified interfaces. There are in general three different categories of approaches to model electrified interfaces within density functional theory (DFT) or density functional theory-based MD (DFTMD) simulations: grand-canonical, counter-ion 10,21-24 /pseudo-atom and finite-field. Grandcanonical approaches are usually parameterized with implicit solvation models where the Helmholtz capacitance (commonly quoted as 20 µF/cm 2 ) is an input quantity rather than a prediction. In the counter-ion/pseudo-atom approaches, ions in electrolyte solution are either out of their equilibrium positions or simply neglected. This renders the difficulty of being certain about the computed capacitance from DFTMD simulations with these methods and make it less useful to guide semi-classical and quantum-mechanical/molecular mechanical (QM/MM) types of approaches. On the other hand, finite-field methods, in particular with constant electric displacement D Hamiltonian, have been shown to be rather useful in investigating the dielectric response of polar fluids, bulk electrolyte solutions, polar surfaces, and the protonic double-layer at insulating metal-oxide/electrolyte interfaces. Despite of that, its implementation for electrified metallic electrodes immersed in electrolyte solution beyond a semi-classical approach is long awaited. In the finite-field treatment of electrified interfaces under periodic boundary conditions, there is only one metal slab, which is polarized by the D field (see Fig. ). The metal slab as a whole remains neutral and so does the electrolyte slab. Since the metal slab is an electronic conductor while the electrolyte slab is an electronic insulator, the (electronic) surface charge density on the metal σ m is related to the applied D field by D = 4πσ m . The resulting potential across the supercell of length L is ∆ϕ = -(D -4πP )L, as guaranteed by the modern theory of polarizatioin 41 P and the Stengel-Spaldin-Vanderbilt constant D Hamiltonian. For electrified interfaces, this (cell) potential ∆ϕ equals to the sum of the potential over two double layers ∆ϕ EDL because the electrolyte is an ionic conductor and the Maxwell electric field over the electrolyte vanishes (on statistical average).