Assignment of excitonic insulators in ab initio theories: The case of NiBr2

The determination of an excitonic insulator phase relies on the relative value of the excitonic binding energy and the fundamental band gap. With the quite unique case of NiBr${}₂$, the authors warn about the risk of running into false positives when using the $G\phantom{\rule{0}{0ex}}W$+BSE scheme to determine if a system displays negative excitonic energies, which are a signature of an excitonic insulator phase. They show that consistency between approximations used in the ground-state and excited-state calculations as well as independence from the starting point are essential features for a proper description of excitonic insulators.