Fig. Two-dimensional structural potential energy surface (PES) spanned by the principal imaginary harmonic modes at the q q q = Γ and q q q = Y wavevectors in Cmcm SnSe as a function of the distortion amplitudes Q Γ and Q Y . The Cmcm structure is a local maximum, corresponding to a hilltop feature, centred around (0, 0), and is connected to two equivalent distortions to the lower-symmetry Pnma minima, identified as dark blue, via the Y -point imaginary mode.