Comment on ‘Gruneisen approach for universal scaling of the Brillouin shift in gases’

I comment on the recent article Grüneisen approach for universal scaling of the Brillouin shift in gases by Liang et al (2022 New J. Phys. 24 103005). While the result of this article indeed provides a parametrization of measured Rayleigh Brillouin scattering spectra, I argue that the association with the Grüneisen effect is fortuitous. The Grüneisen effect is the change of the specific heat with compression of a gas. For the nontrivial case of the van der Waals gas, it predicts the decrease of the velocity of sound with increasing pressure. In solids, which is the context considered in the article by Liang et al, the effect is opposite: an increase of the velocity of sound with pressure.