LAUSR

We report here a giant zero-point energy renormalization of 273 meV in the direct band-gap at $\textbf{K}$ and a 571 meV of blue-shifting in the position of the doubly-degenerate brightest excitonic peak in monolayer hexagonal boron nitride. The non-radiative exciton linewidth is found to be 97 meV at 0K with a large coupling strength of 1.1 eV. This linewidth is found to be mainly dominated by the scattering from the longitudinal optical phonons near the degenerate LO-TO mode, with negligible contributions from other lower branches. Additionally, the band-gap has a temperature dependent slope of -0.53 meVK$^{-1}$, which we found to be in excellent agreement with the reported experimental data on large diameter boron nitride nanotubes. We obtained our results by solving a coupled electron-hole Bethe-Salpeter equation including the lattice vibrational dynamics, purely using ab-initio formalism.