12. However, given that large supercells are likely to be required for accurate results, this "brute force" approach is not feasible. The reduction in 𝜅 latt due to alloying and atomic substitution can be understood conceptually as arising from mass disorder and fluctuations in the strain field introduced by inhomogeneous chemical bonding. The former contribution can be modelled approximately using an isotope-scattering model, and we previously found that this approach provided a remarkably good prediction of the reduced 𝜅 latt in the Pnma Sn(S0.1875Se0.8125) alloy. A significant effect from strain-field fluctuations would be expected when the endpoints in the alloy show significantly different bond strengths. Since the bond strengths are captured by the force constants used to compute the harmonic phonon spectra and thermal conductivities, we can obtain some insight into this by recalculating the 𝜅 latt of the endpoint structures with the masses adjusted to the alloyed element. Taking Pnma Bi2(Se1-xSx)3 as an example, if setting the mass of S to Se in the Bi2S3 structure gives a good reproduction of the 𝜅 latt of Bi2Se3, and vice versa, it would imply that the chemical bonding (force constants) in both phases are similar, and the primary difference in the 𝜅 latt is due to the different chalcogen masses. In this case, we might then expect an isotopescattering model to give a good reproduction of the thermal conductivity of the alloy. To investigate further, we approximate the 𝜅 latt of the three alloys as a function of composition using the method employed in our previous work on the Li(No,Mn,Co)O2 (NMC) alloy system. We calculate the 𝜅 ave of both endpoint structures as the average mass 𝑚 ave of the chalcogen is varied to the other endpoint: