Transverse collective excitations in liquids: Is the phonon theory of liquid thermodynamics correct?

Nonhydrodynamic transverse excitations in liquids exhibit a propagation gap in the long-wavelength region of their dispersion curve. Using molecular dynamics simulations, we investigate the density dependence of the propagation gap width in simple liquids. Based on these findings, we critically analyze the so-called phonon theory of liquid thermodynamics by comparing its analytical predictions with simulation data for supercritical neon (Ne). One contentious aspect of the phonon theory for liquids is the treatment of undamped transverse excitations and their frequency range. We present spectra of transverse propagating modes for supercritical Ne, compare the estimated propagation gaps with theoretical predictions, evaluate the total energy per particle, and discuss the limitations of the phonon theory in accurately describing the simulation results.