In Figure we also show the computed absorption onset at 300 K considering indirect transitions. This simulation includes the effect of electron-phonon coupling, which enables the absorption of a photon across the indirect band gap mediated by the scattering of a phonon. In this case, we see an absorption onset of 1.0 eV, which is significantly closer to the indirect band gap of 1.23 eV at the static lattice value, and in reasonably good agreement with the experimental spectrum. The red-shift of the finitetemperature absorption onset of approximately 200 meV with respect to the static indirect band-gap value can be ascribed to the temperature-dependent renormalisation of the indirect band gap. From this result, we expect the experimental band gap of CaCuP to decrease with increasing temperature as found in most semiconductors. Band gap renormalisations between 50 meV and 200 meV are fairly common when the temperature is elevated from 0 K to 300 K.