Several trends are apparent from the calculations. For a given structure type (i.e. Pnma or R3 ̅ m), the 𝜅 latt decreases with increasing chalcogen mass as expected. More interestingly, for a given chalcogen the average 𝜅 latt of the Pnma structure is consistently smaller than that of the R3 ̅ m phase, by 60-70 %. This is most noteworthy for Bi2Se3, which has been reported in both structure types and for which our calculations predict the Pnma and R3 ̅ m phases to be energetically competitive. Finally, we find that for both structure types the 𝜅 latt is predicted to be anisotropic. In the Pnma phases, the conductivity is largest along the short axes (the a axis for Bi2S3 and the b axes for Bi2Se3 and Bi2Te3; c.f. Table ). The 𝜅 latt is consistently around 80 % smaller along one of the two orthogonal axes, and is 52, 46 and 37 % smaller along the other axis in Bi2S3, Bi2Se3 and Bi2Te3 respectively.