To determine the ground state crystal structure, the relaxed geometries and formation energies of 57 cation arrangements were calculated (all combinations of cations along the M 2 S 2 in a 16 and 32 atom unit cells plus the previously reported structures). Included in the set of structures were the experimentally determined configurations plus the theoretically predicted Cmc2 1 structure. A further energy lowering of 63 meV/atom is found by switching cation positions to a mixed Sn 0.5 Sb 0.5 occupancy along the metal-sulfur chains. The lowest energy arrangement in a 16 atom cell is found when the Sn 2+ and Sb 3+ cations alternate along the M 2 Ch 2 chain (ABAB) (Figure ). We also find an improvement in the agreement of the lattice parameters with experiment (Table ), driven by a contraction of the metal-sulfur bond along the chains. Electrostatics appear to be the main driving force in the reduction in energy with a 26 meV/atom reduction in the Madelung energy.