Fig. 6. Phonon dispersion ħωj(q) and density of states g(ħω) (DoS) of cubic crystalline NH₃ from DFT calculations (see ESI,† for details). Panel (a) shows the full DoS and highlights four distinct groups of modes, viz. the “external” (pseudo-)acoustic modes formed from rigid-molecule translations and rotations (blue), and the “internal” modes formed from the NH₃ bending (red), H–N–H “scissoring” (orange), and N–H symmetric and antisymmetric stretch vibrations (purple). Panel (b) shows the phonon dispersion and DoS from 0–700 cm⁻¹ corresponding to the blue region in (a).