alloys*

Two-dimensional (2D) post-transition metal chalcogenides (PTMCs) have attracted attention due to their suitable bandgaps and lower exciton binding energies. Of the predicted 2D PTMCs, GaSe has been reliably synthesized and experimentally characterized. Despite this fact, results vary depending on which density functional theory (DFT) functional is used. In an attempt to correct these discrepancies, we employed Diffusion Monte Carlo (DMC) to calculate the ground and excited state properties of GaSe because DMC has a weaker dependence on the trial wavefunction. We benchmark these results with experimental data, DFT and many-body perturbation theory (GW-BSE). We confirm that monolayer GaSe is an indirect gap semiconductor (Γ-M) with a quasiparticle gap in close agreement with experiment and low exciton binding energy. We also benchmark the optimal lattice parameter and cohesive energy with DMC and various DFT methods. In addition, we studied GaSe_1-xS_x alloyed structures (including Janus GaSSe) using QMC and tested the transferability of their optimized Jastrow parameters between different alloys. We aim to present a terminal theoretical benchmark for pristine monolayer GaSe and alloys, which will aid in the further study of 2D PTMCs and alloys using DMC methods.