  The theoretical dispersion curves $n(\omega)$ (refractive index versus
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.
