Fundamental gap of fluorographene by many-body GW and fixed-node diffusion Monte Carlo methods

Fluorographene (FG) is a promising graphene-derived material with a large bandgap. Currently existing predictions of its fundamental gap (Δ_f) and optical gap (Δ_opt) significantly vary when compared with experiment. We provide here an ultimate benchmark of Δ_f for FG by many-body GW and fixed-node diffusion Monte Carlo (FNDMC) methods. Both approaches independently arrive at Δ_f ≈ 7.1 ± 0.1 eV. In addition, the Bethe-Salpeter equation enabled us to determine the first exciton binding energy, E_b = 1.92 eV. We also point to the possible misinterpretation problem of the results obtained for gaps of solids by FNDMC with single-reference trial wave functions of Bloch orbitals. We argue why instead of Δ_opt, in the thermodynamic limit, such an approach results in energy differences that rather correspond to Δ_f, and we also outline conditions when this case actually applies.