In the high-symmetry cubic Fm 3m structure both the tetrel and chalcogen atoms occupy six-fold coordinated octahedral sites. The rhombohedral R3m structure is derived from the rocksalt structure by a small displacement of the cations from their ideal positions, which results in both atoms forming three short and three long bonds. The Pnma phase can be considered a further distortion to the rocksalt structure in which every other unit cell along the b axis is offset, resulting in a pseudo 2D layered structure in which the Ge and S atoms each form two short and one long bond. This structure enables the Ge 2+ cations to have an active 4s 2 lone pair, which projects into the interlayer space and supports a van der Waals interaction between adjacent layers. Fig. Structures of the Pnma (a), R3m (b), and rocksalt (Fm 3m, c) phases of GeS and GeSe. Purple and yellow spheres denote the Ge and chalcogen atoms, respectively. The R3m and Fm 3m structures are shown in the conventional cells. These figures were generated using VESTA. The optimized lattice constants are compared to previous reports in Table in the ESI. † The lattice parameters for the Pnma phases are in good agreement with previous measurements. The lattice parameters for rocksalt and R3m GeSe are underestimated by 3.5% and 2.5% compared to experiments, which we attribute to the measurements in Refs 54 and being carried out at finite temperature and therefore including an appreciable amount of thermal expansion compared to our "athermal" DFT optimisations. There are some discrepancies between our calculations on the R3m and rocksalt phases with other theoretical work, which we attribute primarily to the tendency of the PBE functional employed in these studies to overestimate lattice constants and cell volumes compared to PBEsol+D3.