Constant-Potential Modeling of Electrical Double Layers Accounting for Electron Spillover

Constant-potential molecular dynamics (MD) simulations are indispensable for understanding the structure, capacitance, and dynamics of electrical double layers (EDLs) at the atomistic level. However, the classical constant-potential method, relying on the so-called ``fluctuating charges'' to keep electrode equipotential, overlooks quantum effects on the electrode and always underestimates EDL capacitance for typical metal electrode and aqueous electrolyte interfaces. Here, we propose a constant potential method accounting for electron spillover on the outermost nuclei of the electrode. For EDLs at Au(111) electrodes, our MD simulation reveals bell-shaped capacitance curves in magnitude and shape both quantitatively consistent with experiments. It unveils the electrode-polarization-dependent local electric fields, agreeing with experimental observations of redshift vibration of interfacial water under negative polarization and predicting a blueshift under positive polarization, and further identifies geometry dependence of two timescales during charging.