Stronger correlations are apparent among the two sets of materials in this work with vacancy migration barriers less than and greater than 600 meV, respectively, although apart from the migration barriers, there is no obvious feature differentiating the materials in these two sets. As is evident in Figure Muy et al. argued that band centers are correlated with migration barriers because lower frequencies are associated with larger thermal displacements of migrating cations. This argument appears to be predicated on the assumption that low-frequency phonon modes displace atoms in directions that facilitate ion migration. This suggests a possible explanation for why correlations are observed among some sets of materials but not others: if low-frequency phonon modes differ in the extent to which they displace ions in favorable directions, their frequencies (and therefore the materials' overall phonon band centers) may not correlate as strongly with migration barriers. A similar argument was made in Sagotra et al.: for the materials in ref 30, the atoms comprising the stationary anionic sublattice through which Li ions migrate are heavier than Li, so low-frequency modes likely involve vibrations of this sublattice. If the sublattice vibrates in directions that facilitate migration (e.g. widening of migration pathways), thermal excitation of these modes may lead to more facile Li migration. For some materials considered in this work, the masses of atoms comprising the anionic sublattice are less than that of the migrating cation, while in others, they are greater.