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

In this work we perform a detailed first-principles analysis of the electronic and optical properties of NiBr2 within the state-of-the-art GW+BSE scheme to determine whether this system displays negative excitonic energies, which would identify it as an (half) excitonic insulator. Particular attention is paid to the convergence of the GW band structure and to the consistency between approximations employed in the ground-state calculations and approximations employed in the linear response calculations. We show that these two issues play a crucial role in identifying the excitonic nature of NiBr2.