Whereas the phonon dispersion curves of both Pnma and R3m structures show real frequencies across the Brillouin zone, the two rocksalt structures show prominent imaginary modes at the q q q = Γ wavevector, indicating that the higher-symmetry cubic phase is dynamically unstable. The position of the imaginary modes further indicates that the energy-lowering distortion can be represented in the primitive cell and therefore likely corresponds to a cation off-centering to the R3m structure. This in turn suggests that the high-temperature rocksalt structure observed in experiments may be a crystallographic average over locallydistorted R3m domains. The predicted dynamical stability of Pnma GeS/GeSe and R3m GeSe are consistent with previous studies, as is the instability of rocksalt GeSe. There is, however, an interesting contrast with the analagous rocksalt SnS and SnSe. Rocksalt SnS is dynamically unstable under ambient conditions, but the imaginary mode lies at q q q = X rather than q q q = Γ, indicating a different type of energy-lowering distortion. In contrast to both SnS and GeSe, rocksalt SnSe is dynamically stable. The calculations in Ref. predicted R3m GeS to be dynamically unstable with a small imaginary mode along the Γ → X and Γ → L directions in the phonon dispersion. We attribute this discrepancy to an overestimation of the unit-cell volume with PBE, which has been shown in some cases to introduce unphysical imaginary modes into phonon spectra. To rule out possible technical issues, we tested a series of second-order supercell expansions containing 300 -648 atoms, but did not observe imaginary modes in any of these calculations (Fig. ).