A combination of computational techniques were used to investigate the structure of Sn 2 SbS 2 I 3 over different length and timescales. The base calculations were performed using density functional theory (DFT), using the projector augmented wave method as implemented in the Vienna ab initio simulation package (VASP). For geometry optimization the dispersioncorrected optB86b-vdW functional was chosen as it accounts for the van der Waals interactions commonly observed in layered materials and has been shown to be important for obtaining accurate forces in these compounds. For the calculation of the electronic band structure of the athermal ground-state crystal structure, the screened hybrid functional HSE06 was used with the inclusion of spin orbit coupling (HSE06 + SOC). For each structure considered in this work, plane-wave cutoff energy and k-mesh spacing were converged to a total energy differences less than 1 meV/atom, with a 40% increase in planewave cutoff energy during relaxations to avoid the introduction of Pulay stress. The shift current for these structures was calculated using the Wannier interpolation scheme as implemented in Wannier90 (calculation detail provided in the Supporting Information). To investigate the extent of cation disorder in Sn 2 SbS 2 I 3 , all combinations of cations along the M 2 S 2 in a 16 and 32 atom unit cells (55 structures) were generated using the integrated cluster expansion toolkit (ICET). The energies (normalised per atom) and bandgaps were calculated using the optB86b-vdW functional, and two separate cluster expansions (CE)