Temperature dependence of the proton kinetic energy in water between 5 and 673 K

The proton mean kinetic energy Ke(H) in water was calculated versus temperature at 5-673 K using the harmonic approximation and assuming decoupling between translation, rotation-libration, and internal vibrations. The input data were taken from optical and inelastic neutron scattering measurements. The effect of H-bonding is discussed together with the role of zero-point motion and its effect on the proton dynamics in water. Good agreement was obtained with the results of recent inelastic and deep inelastic neutron scattering measurements of Ke(H). The large discrepancies between the calculated values and previous measurements for supercooled water at 269-271 K and for water at 300 K is discussed. Unexpectedly, Ke(H) seems to reveal a nearly continuous behavior when passing from solid to liquid and to the supercritical phase. The calculated kinetic energy of the oxygen-atom in water versus T is also reported.