  Gap plasmonics deals with the properties of surface plasmons in the narrow
region between two metallic nanoparticles forming the gap. For sub-nanometer
gap distances electrons can tunnel between the nanoparticles leading to the
emergence of novel charge transfer plasmons. These are conveniently described
within the quantum corrected model by introducing an artificial material with a
tunnel conductivity inside the gap region. Here we develop a methodology for
computing such tunnel conductivities within the first-principles framework of
density functional theory, and apply our approach to a jellium model
representative for sodium. We show that the frequency dependence of the tunnel
conductivity at infrared and optical frequencies can be significantly more
complicated than previously thought.
