The phonon dispersion and density of states (DoS) curves of the Pnma, R3m and rocksalt phases of GeS are shown in Fig. . The unit cell of Pnma phase contains n a = 8 atoms, and the primitive cells of the R3m and rocksalt phases contain n a = 2 atoms, resulting in 3n a = 24 and 3n a = 6 branches at each wavevector q q q in the respective phonon dispersions. The phonon dispersion of Pnma GeS, with the most phonon bands, is notably more complex than those of the R3m and rocksalt phases. The lower symmetry of the R3m phase and consequent lower band degeneracy also results in a more complex phonon spectrum than for the rocksalt structure. The atom-projected DoS (PDoS) curves for Pnma and R3m GeS show that the lower-and higher-frequency modes are dominated by motions of heavier Ge and lighter S, respectively. Pnma GeS also shows a clear separation between the 12 lower-and higherfrequency modes and a prominent "phonon bandgap". In contrast, while the PDoS of rocksalt GeS also shows a low-frequency feature dominated by Ge motion, there is considerable overlap between Ge and S at higher frequencies.