  We combine ab initio simulations and Raman scattering measurements to
demonstrate explicit anharmonic effects in the temperature dependent dielectric
response of a NaCl single crystal. We measure the temperature evolution of its
Raman spectrum and compare it to both a quasi-harmonic and anharmonic model.
Results demonstrate the necessity of including anharmonic lattice dynamics to
explain the dielectric response of NaCl, as it is manifested in Raman
scattering. Our model fully captures the linear dielectric response of a
crystal at finite temperatures and may therefore be used to calculate the
temperature dependence of other material properties governed by it.
