  We employ systematic calculations based on density functional theory to model
the reflectivity of CaS and MgS in the infrared region. We show that in
addition to the modeling using the harmonic approximation, an accurate spectral
description requires the inclusion of anharmonic effects. Due to their
conceptual simplicity, CaS and MgS are excellent systems for the explicit
consideration of the anharmonicity, which we include here using a perturbative
approach up to three-phonon scattering processes, and the consideration of
isotopic disorder. All physical quantities, such as Born effective charges and
dielectric constant, necessary for the calculation of the reflectivity within
the Lorentz model are extracted from our first-principles computations. To
validate our predicted optical and transversal modes, and reflectivity spectra,
we compare them to available experimental results. We find that the overall
agreement is good, which supports the importance of the inclusion of anharmonic
terms in the modeling of optical properties in the infrared region.
