Our predicted 300 K κ latt = 4.53 W m -1 K -1 for R3m GeSe is similar to the 4.88 W m -1 K -1 obtained in previous calculations, 100 whereas we attribute the lower values obtained from the calculations in Ref. to an overestimation of the lattice constants with PBE. We were only able to find experimental measurements on polycrystalline R3m (GeSe) 0.9 (AgBiSe 2 ) 0.1 , which report lattice thermal conductivities of 0.74, 0.38 and 0.47 W m -1 K -1 at 300, 578 and 723 K, respectively. While these are much lower than our predicted 4.53, 2.36 and 1.95 W m -1 K -1 , we attribute this to the established impact of alloying on the κ latt . Having shown that our predicted κ κ κ latt are in good agreement with comparable measurements, we used the analysis techniques developed in our previous work to further investigate the origin of the differences between the systems. We first rewrite the κ κ κ latt as the product of a harmonic term and a weighted-average phonon lifetime τ CRTA according to: