Lattice-dynamics calculations were performed on the primitive cell of 1, obtained from the Cambridge Structural Database (CCDC: 1416543), using periodic density-functional theory (DFT) as implemented in VASP 5.4.4. The PBE functional with the DFT-D3 semiempirical dispersion correction was employed to model the electron exchange and correlation. The ion cores were modelled with projector augmented-wave (PAW) pseudopotentials, with a 4f-in-core potential for Dy(III) used to avoid issues with treating the strongly-correlated f electrons using semi-local DFT. We used a plane-wave basis set with an energy cut-off of 800 eV and Γ-point Brillouin zone sampling to model the valence electronic structure, with both parameters determined via explicit convergence testing. Starting from the 128-atom primitive cell of the published X-ray structure of 1, the atomic positions and unit-cell parameters were optimised to tight tolerances of 10 -8 eV on the electronic total energy and 10 -2 eV Å -1 on the forces. Lattice-dynamics calculations were performed using the supercell finite-difference method implemented in the Phonopy and Phono3py 42 codes. A 2×2×1 supercell with 512 atoms was employed to determine the second-order force constants in Phonopy, while the third-order force constants were determined for the primitive cell (i.e. a 1×1×1 cell) using Phono3py, and the phonon frequencies and linewidths were evaluated on uniform 2×2×2, 3×3×3, 4×4×4 and 5×5×5 q-point grids using Fourier interpolation.