Finally, the spectrum of Cmcm SnSe in Fig. shows two imaginary phonon modes at the q q q = Γ and Y wavevectors. These modes are linked to the soft-mode phase transition to the Pnma phase. Mapping the instability as a function of the distortion amplitudes Q Γ and Q Y (Fig. ) shows that the Cmcm phase is a local maximum on the two-dimensional potential-energy surface (PES) spanned by the principal imaginary modes, connecting two minima located along q q q = Y . These minima correspond to two equivalent distortions of the Cmcm structure to the lowersymmetry Pnma phase. The minima are relatively shallow with respect to the hiltop at (Q Γ = 0, Q Y = 0), and at temperatures above the phase transition the two minima can interconvert sufficiently quickly that experimental techniques that probe the longrange structure (e.g. X-ray diffraction) will effectively "see" the Cmcm phase as an average structure. The soft-mode transition in Cmcm SnSe, and its impact on the structure and lattice dynamics, has been the subject of a number of previous studies. We also note that the spectrum of GeSe has a triply-degenerate imaginary optic mode at the q q q = Γ wavevector, which becomes doubly-degenerate when a non-analytical correction is applied, and this likely indicates a distortion to the R3m phase. However, we did not pursue this further.