Boundary conditions in periodic density functional calculations of insulating materials

Abstract

Ewald's method yields internal electric fields in a polar material consisting of local and macroscopic components. The macroscopic component is a priori undefined. It depends on the boundary conditions imposed at the surface of the macroscopic medium. The boundary conditions ordinarily used in periodic electronic-structure calculations correspond to assuming that the modeled material is embedded in a metal connected to the ground and are equivalent to considering the macroscopic components of the internal electric fields to be null. These boundary conditions are commonly referred to as closed-circuitboundary conditions. In this study, we discuss and use an alternative type of boundary conditions, suitable for describing the most common physical situation of a polar material with the surface exposed to the vacuum, that is, under open-circuit conditions. We implement these boundary conditions in a periodic density functional approach and use the resulting energy scheme to study NaCl, MgO, and BaO. A comparison with the experiments shows that periodic density functional calculations employing the open-circuit boundary conditions give an excellent description of both dynamical and dielectric properties of polar materials.