From the ultraviolet to the mid-infrared region, light-matter interaction mechanisms in semiconductors progressively shift from electronic transitions to phononic resonances and are affected by temperature. Here, we present a unified temperature-dependent treatment of both electrons and phonons entirely from first principles, enabling the prediction of a full-spectrum dielectric function with ${\mathrm{CeO}}_2$ as the prototype material. At elevated temperatures, ab initio molecular dynamics is employed to find thermal perturbations to electronic structures and construct effective force constants describing potential energy surface. Four-phonon scattering and phonon renormalization are included in an integrated manner in this approach. Our first-principles-calculated refractive index of ${\mathrm{CeO}}_2$ agrees well with measured data from literature and our own temperature-dependent ellipsometer experiment.

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• Dielectric properties
• Electronic structure
• Lattice dynamics
• Light-matter interaction
• Semiconductors
• Ellipsometry
• First-principles calculations