Simulating an electrochemical interface using charge dynamics

We present a simple classical method for treating charge mobility in metals adjacent to liquid solutions. The method, known as electrode charge dynamics, effectively bridges the computational gap between ab initio calculations on small metal clusters and large-scale simulations of metal surfaces with arbitrary geometry. We have obtained model parameters for a copper (111) metal surface using high-level quantum-mechanical calculations on a 10-atom copper cluster. We validated the model against the classical image-charge result and ab initio results on an 18-atom copper cluster. The model is used in molecular dynamics simulations to predict the structure of the fluid interface for neat water and for aqueous NaCl solution. We find that water is organized into a two-dimensional ice-like layer on the surface and that both Na+ and Cl- are strongly bound to the copper. When charging the metal electrode, most of the electrolyte response occurs in the diffuse part of the double layer.