Infrared reflectance, transmittance and emittance spectra from first-principles

Density functional theory (DFT) is considered a predictive approach to determine anharmonic phonon-phonon interactions in crystals. Such a computational tool is a necessary ingredient, for example, of the first principles computational methods recently developed to determine phonon thermal transport in real systems. In this context, finding measurable quantities that can provide an independent benchmark for theoretical approaches is of paramount importance. Surprisingly, DFT has been rarely used to interpret the anharmonic features observable in infra-red (IR) reflectance, transmittance and emittance spectra even though they provide a relatively direct probe to anharmonic properties in heteropolar materials and can be used to directly determine the anharmonic phonon self-energy of the optically active mode. I will show the influence of anharmonic effects on IR spectra of MgO, which is chosen as a test material because of the availability of different kinds of radiative properties measured experimentally and I will explain the limit of validity of a perturbative (multi-phonon) approach.