A many-body characterization of the fundamental gap in monolayer CrI₃

The many-body fixed-node and fixed-phase spin-orbit diffusion Monte Carlo (DMC) methods are applied to accurately predict the fundamental gap of monolayer CrI₃ – the first experimentally realized 2D material with an intrinsic magnetism. We numerically show that the same value is obtained at the thermodynamic limit when using neutral promotions and the standard quasiparticle definition with ionization potential and electron affinity. The obtained value of 2.9(1) eV agrees well with the highest peak in the optical spectroscopy measurements and a previous GW result. Additional analysis of density matrix differences using configuration interaction calculations explains why a single-reference trial wave function should produce an accurate excitation. We find that the account of electron correlations is more crucial than the spin-orbit effects in determining the fundamental gap. These results serve as another case of benchmark DMC application in 2D materials and emphasize the importance of using beyond-DFT methods in this class of systems.