  Molecular-scale dynamics in sub- to super-critical water is studied with
inelastic X-ray scattering and molecular dynamics simulations. The obtained
longitudinal current correlation spectra can be decomposed into two main
components: a low-frequency (LF), gas-like component and a high-frequency (HF)
component arising from the O--O stretching mode between hydrogen-bonded
molecules, reminiscent of the longitudinal acoustic mode in ambient water. With
increasing temperature, the hydrogen-bond network diminishes and the spectral
weight shifts from HF to LF, leading to a transition from liquid-like to
gas-like dynamics with rapid changes around the Widom line.
