where κ αα are the diagonal elements of the κ κ κ tensor and correspond to transport along the principal x, y and z directions. However, the thermal conductivity is only isotropic (i.e. all three diagonal components are equal) for the Fm 3m phase, whereas for the R3m phase κ xx = κ yy ̸ = κ zz and for the orthorhombic Pnma and Cmcm phases κ xx ̸ = κ yy ̸ = κ zz . For each compound, we show the separate temperature dependencies of the κ xx , κ yy and κ zz in the ESI † and the values at 300 K are included in Table . The 300 K κ iso = 1.36 W m -1 k -1 calculated for Pnma SnSe is in line with our previous calculations of 1.28 and 1.58 W m -1 K -1 . We attribute the differences primarily to the different supercell used for the force-constant calculations compared to Ref. and to the omission of a dispersion correction compared to Ref. . Our previous calculations on Cmcm SnSe yielded a κ iso of 0.33 W m -1 K -1 at 800 K, and we obtain a similar 0.36 W m -1 K -1 here. Two separate reports give the κ iso pf Pnma GeSe to be 2.63 and 1.77 W m -1 K -1 at 300 K, and our calculated value of 2.35 W m -1 K -1 falls within this range. Our calculated roomtemperature κ iso = 4.36 W m -1 K -1 for R3m GeTe is around 30% higher than the experimentally-measured ∼ 3.4 W m -1 K -1 . The thermal conductivity of Fm 3m GeTe has been measured as ∼1.5 W m -1 K -1 and predicted to be 0.75-1.25 at 700 K , the latter of which compares well to the value of 0.72 W m -1 K -1 from our calculations. The room-temperature κ iso of Fm 3m SnTe has been measured around 2.5 W m -1 K -1 at 300 K, but our predicted value of 5.01 W m -1 K -1 is some 2× larger. We should note, however, that comparison to experiments should be treated with caution since measurements are inherently sensitive to sample preparation and to environmental factors such as partial oxidation at elevated temperature.