In this work we study how different atomic configurations, and electronic correlation modify the exciton dispersion in hexagonal boron nitride. We use two different approaches, ab-initio DFT plus many-body perturbation theory and tight-binding.
We found that in the case of AB-stacking electronic correlation inverts the nature of the optical gap, from indirect to direct respect to the single-particle picture.

The paper is available here: https://arxiv.org/abs/1806.06201