The MOF structures show several phonon modes with low frequency, <2 THz, which often give rise to NTE. Phonon modes of this frequency are attributed to collective vibrational modes, which can be visualized using the Molden 47,48 software package with the normal mode data files available online from our data repository at . For DUT-60 we can confirm these low-energy vibrational modes correspond to "trampoline"-like vibrations of the ligands that are also NTE-contributing modes in MOF-5. Phonons that soften in response to contraction of the lattice have negative Grüneisen parameters, which directly contribute to the NTE observed in the material. Please note, the softening described here relates to phonons shifting to lower frequencies. A comparison of the phonon density of states at different volumes, corresponding to the average MOF unit cell volume at 40 and 300 K highlight which of these low-energy modes give rise to NTE (Figure ). For each MOF, we find one or several peaks that display softening upon contraction, which contribute to the NTE observed in these systems.