  The energy gap of simple band insulators like GaAs is a strong function of
temperature due to the electron-phonon interactions. Interestingly, the
perturbation from zero-point phonons is also predicted to cause significant (a
few percent) renormalization of the energy gap at absolute zero temperature but
its value has been difficult to estimate both theoretically and, of course,
experimentally. Given the experimental evidence [Bhattacharya, et al., Phys.
Rev. Lett. 114, 047402 (2015)] that strongly supports that the exponential
broadening (Urbach tail) of the excitonic absorption edge at low temperatures
is the manifestation of this zero temperature electron-phonon scattering, we
argue that the location of the Urbach focus is the zero temperature
unrenormalized gap. Experiments on GaAs yield the zero temperature bare energy
gap to be 1.581 eV and thus the renormalization is estimated to be 66 meV.
