Electronic density functional theory in the grand canonical ensemble, electrochemistry, and the underpotential deposition of Cu/Pt(111)

The study of electrochemical systems within electronic density functional theory requires the handling of non-neutral electronic systems in the plane-wave basis in order to accurately describe charged metallic surfaces; this can be accomplished in joint density functional theory by adding an electrolyte with Debye screening \footnote{K. L. Weaver and T. A. Arias (under preparation)}. This capability opens up the opportunity to work in the grand canonical ensemble at fixed chemical potential $\mu$ for the electrons, which corresponds directly to the experimental setting in electrochemistry. We present efficient techniques for electronic density functional calculations at fixed $\mu$, and demonstrate the improvement in predictive power over conventional neutral calculations using the underpotential deposition of Cu/Pt(111) as an example: for the first time, we calculate absolute voltages for electrochemical processes in excellent agreement with experiment, instead of voltage shifts alone.