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

Many-body fermionic Diffusion Monte Carlo (DMC) methods are applied to accurately predict the fundamental gap of the monolayer ferromagnet CrI 3 . The fundamental gap obtained, Δ f = 2.9(1) eV, agrees well with the highest peak in optical spectroscopy measurements and a previous G W result. We numerically show that the same value of Δ f is obtained in the thermodynamic limit using both neutral promotions and the standard definition of Δ f based on the ionization potential and electron affinity. Analysis of the differences between density matrices of natural orbitals obtained from configuration interaction calculations explains why a single-reference trial wave function can produce an accurate excitation. We find that accounting for electron correlation is more crucial than accounting for spin-orbit effects in determining Δ f . These results highlight the power of DMC for benchmarking 2D material physics and emphasize the importance of using beyond-DFT methods for studying 2D materials.