The term in the summand is calculated from the C λ and ν ν ν λ , both of which are obtained within the harmonic approximation, whereas the τ CRTA are derived from the τ λ and depend on the anharmonic phonon interactions. Both the harmonic sum κ κ κ/τ CRTA and the τ CRTA are normalised such that they are directly comparable between systems in the same way as the macroscopic κ κ κ. The C λ are a relatively shallow function of the phonon frequency and quickly saturate to the Dulong-Petit limit of N A k B per mode at mid-to-high temperature, and thus differences in the harmonic sum largely reflect differences in the ν ν ν λ . The τ CRTA are effectively a weighted average over the τ λ , weighted toward the modes that make the largest contributions to the thermal conductivity. A comparison of the τ CRTA to the frequency spectra of the phonon lifetimes for each of the ten systems at 300 K is provided in the ESI. † Finally, we note that in the formulation in Eq. 9 the harmonic sum is a tensor, and an analagous scalar average to κ iso , denoted κ κ κ/τ CRTA iso , can be taken for easier comparison between systems.