  Developing microscopic understanding of the thermal properties of liquids is
challenging due to their strong dynamic disorder, which prevents
characterization of the atomic degrees of freedom. There have been significant
research interests in the past few decades to extend the normal mode analysis
for solids to instantaneous structures of liquids. However, the nature of
normal modes that arise from these unstable structures is still elusive. In
this work, we explore the instantaneous eigenmodes of dynamical matrices of
various Lennard-Jones argon liquid/gas systems at high temperatures and show
that the normal modes can be interpreted as an interpolation of T \to \infty
(gas) and T = 0 (solid) mode descriptions. We find that normal modes become
increasingly collisional and translational, recovering atomistic gas-like
behavior rather than vibrational with increase in temperature, suggesting that
normal modes in liquids may be described by both solid-like and gas-like modes.
