Surface Potential at the Air−Water Interface Computed Using Density Functional Theory

An accurate prediction of the surface potential (ϕ) at the air−water interface is critical to calculating ion hydration free energies and electrochemical half-cell potentials. Using density functional theory (DFT), model interfacial configurations, and a theoretical definition of ϕ, we report a value of +3.63 V at 0.92 g/cm 3 water density. A maximally localized Wannier function analysis confirms that ϕ is dominated by molecular quadrupole (or “spherical second moment”) contributions. We find that the predicted surface potential depends on computational details and conclude that standard DFT codes and the existing theoretical definition of ϕ does not yield surface potentials directly comparable to existing experiments.