The dispersion of Pnma GeSe also displays a prominent "band gap" from ∼3.5-4 THz (Fig. ). This appears to be a feature of the Pnma structure, and is most prominent in GeSe and GeTe but also evident in SnSe and SnTe. In the case of SnSe, the gap marks the point where the relative contributions of the tetrel and chalcogen atoms reverse. The spectrum of R3m GeTe also shows a notable reduction in the DoS at mid frequencies, but this is not seen in the spectrum of R3m SnTe, suggesting that the nature of the tetrel atom plays a role in the gap in this structure type. The absence/lower density of modes in the gap range implies that there is no/little contribution to the lattice thermal conductivity at these frequencies, a point to which we return below.