Calculations were performed using plane-wave density-functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP) code. Initial structures of π SnS and SnSe were taken from previous work and optimised to tight tolerances of 10 -8 eV on the total energy and 10 - eV Å -1 on the forces. PBEsol+D3 was used to describe electron exchange and correlation, as this has been shown to provide an accurate description of the structure and dynamics of these materials. Projectoraugmented wave (PAW) pseudopotentials were used to describe the ion cores, with the valence configurations: Sn -5s 2 5p 2 , S -3s 2 3p 4 , and Se -4s 2 4p 4 . A plane-wave basis with a kineticenergy cutoff of 600 eV was employed with a 4 × 4 × 4 Γ-centred Monkhorst-Pack k-point mesh to model the valence electrons.