As for the electrical properties, the averaged κ latt is likely to be of most relevance to thermoelectric devices. However, Pnma SnS and SnSe both show strongly-anisotropic thermal transport, which is an important contributor to the "headline" ZT in singlecrystal SnSe, 37 so we also examined the anisotropy in the κ κ κ latt of the four systems examined here (Table , Figs ). The symmetry of the R3m structure is such that κ xx = κ yy ̸ = κ zz , and the independent κ xx and κ zz are similar and close to the average. For the Pnma structures, on the other hand, we predict strong anisotropy such that the lowest κ latt is obtained along the layered b axes (κ yy ) and the highest along the strongly-bonded c axes (κ zz ). Indeed, the anisotropy is so large that while Pnma and R3m GeS are predicted to have similar averaged κ latt of 1.23 and 1.11 W m -1 K -1 , respectively, at 800 K, the κ latt of the Pnma phase is an average over three values that range from 0.50 W m -1 K -1 along the b direction to 2.35 W m -1 K -1 along the c axis.