Agreement of classical Kubo theory with the infrared dispersion curves n(ω) of ionic crystals

The theoretical dispersion curves n(ω) (refractive index vs. frequency) of ionic crystals in the infrared domain are expressed, within the Green-Kubo theory, in terms of a time correlation function involving the motion of the ions only. The aim of this paper is to investigate how well the experimental data are reproduced by a classical approximation of the theory, in which the time correlation functions are expressed in terms of the ions orbits. We report the results of molecular-dynamics (MD) simulations for the ions motions of a LiF lattice of 4096 ions at room temperature. The theoretical curves thus obtained are in surprisingly good agreement with the experimental data, essentially over the whole infrared domain. This shows that at room temperature the motion of the ions develops essentially in a classical regime.