The Γ 𝜆 are computed as the imaginary part of the phonon self-energies from a perturbative expression using the harmonic frequencies and eigenvectors and the third-order force constantsfull details of this method are given in Ref. . (𝜅 𝑥𝑥 + 𝜅 𝑦𝑦 + 𝜅 𝑧𝑧 ). We calculate a 𝜅 ave of 0.9 for Pnma Bi2S3, which compares favorably to the experimental value of 0.87 W m -1 K -1 . Our predicted 𝜅 ave for R3 ̅ m Bi2Se3 and Bi2Te3 are 1.82 and 0.88 W m -1 K -1 . The value for Bi2Se3 again compares favorably to the measured value of 1.7 W m -1 K -1 quoted in Ref. 15. On the other hand, the latter significantly underestimates the values of 1.28 and 1.37 W m -1 K -1 measured for Bi2Te3. The reasons for this discrepancy are not clear. Bi2Se3/Bi2Te3 (c.f. Table ), and that in the R3 ̅ m phases 𝜅 𝑥𝑥 = 𝜅 𝑦𝑦 by symmetry.