As for the electronic band structure, the phonon dispersion shows notable anisotropy between the in-plane and out-of-plane directions. The inclusion of NAC has a particularly striking effect along the out-of-plane direction between 26-30 THz where the continuity of the band is broken leading to a marked flattening of the band. With the exception of the three acoustic modes along Γ-Z, the bands along the X-P and Γ-Z directions, which correspond to the z direction in real space, are almost completely flat (i.e. dispersionless). As the group velocity is calculated from the band curvature, this flattening is expected to lower the velocity, which would favour low lattice thermal conductivity. The effect of NAC is less pronounced in the xy plane, but flattening of the modes be seen between 8-12 THz and 16-24 THz along Γ-X and Γ-N, where the dispersion curvature levels out close to the high-symmetry wavevectors.