band of higher-frequency vibrations around 24 THz and a final set of vibrations between 27–30 THz.

There are 204 separate modes $i$ for any wave vector $\mathbf{k}$. The two highest-frequency bands (24 and 27–30 THz) consist of 48 individual modes, the band between 9–11.5 THz consists of 60 individual modes, and the broad band from 0–8 THz consists of the other 96 modes. We discuss eigenvectors associated with these bands in the next subsection.

b shows the dispersion curves with colours used to represent the values of the associated mode Grüneisen parameters $\gamma_{\mathbf{k},i}$ (red for negative and blue for positive values respectively). It is interesting to note that the dispersion diagram is divided into large bands of negative or positive values of mode Grüneisen parameters. This feature stands in contrast to other recently studied systems such as Cu$_2$O [@Leila_Cu2O], ScF$_3$ [@ScF3_phonons], Zn(CN)$_2$ [@Hong_Zn(CN)2] and MOF-5 [@Leila_MOF5], where phonons with significantly different values of mode Grüneisen parameters can exist at similar frequencies due to the significant dependence of the NTE phonon eigenvectors on wave vector. The strongest contributions to NTE come from phonons around 0.7–1 THz ($\gamma_{\mathbf{k},i}\sim-9$ on average, with values of $\sim-15$ at wave vectors near the Brillouin zone points 