Fig. 5. Phonon dispersion ħωj(q) and density of states g(ħω) (DoS) of NaCl from DFT calculations (see ESI,† for details). The dispersion is shown along the wavevector path 
. There are 3na = 6 bands at each q, but the high symmetry of the cubic spacegroup means that a pair of acoustic modes and a pair of optic modes are degenerate along this path; the singly-degenerate bands are shown in red, and the doubly-degenerate bands in blue. The DoS is shown in black and the projections onto the Na and Cl atoms are shown in yellow and green, respectively.
