Ab initio finite-temperature excitons.

The coupling with the lattice vibrations is shown to drastically modify the state-of-the-art picture of the excitonic states based on a frozen-atom approximation. The zero-point vibrations renormalize the bare energies and optical strengths. Excitons acquire a nonradiative lifetime that decreases with increasing temperature. The optical brightness turns out to be strongly temperature-dependent such as to induce bright to dark (and vice versa) transitions. The finite-temperature experimental optical absorption spectra of bulk Si and hexagonal BN are successfully explained without using any external parameter.