Representative images of the four structure types are shown in Fig. . GeSe adopts the disordered orhorhombic Pnma structure at low temperature but undergoes a transition to the highlysymmetric Fm 3m (rocksalt) structure at T ≈ 920 K. GeTe adopts the rhombohedral R3m phase below 690 K and transitions to the Fm 3m phase above this temperature. The Pnma phase of GeTe is also reported to be accessible under applied pressures in the range of 15-37 GPa. SnTe behaves similarly to GeTe but the transition from the rhombohedral to the cubic phase occurs below 100 K. A bulk orthorhombic phase is also reported to be accessible under pressure, and has been observed as nanoscale defects in cubic SnTe under ambient conditions. Finally, SnSe adopts the Pnma phase at low temperature but undergoes a phase transition to a higher-symmetry orthorhombic Cmcm phase around 750 K, the latter of which has been shown to display strong phonon anharmonicity thought to contribute to its exceptional thermoelectric performance. The optimised lattice parameters of each of the ten structures are shown in Table . The parameters of all the low-temperature phases are within 5 % of literature values. Larger errors are obtained for the high-temperature phases, with the calculations predicting an 11% reduction in the cell volume of Cmcm SnSe. This is expected given that standard DFT optimisations yield the athermal equilibrium structures, and we therefore attribute the larger % differences for the high-temperature structures to a larger degree of thermal expansion in the measured structures.