Temperature dependence of the zero point kinetic energy in ice and water above room temperature

By means of Deep Inelastic Neutron Scattering we determined the temperature dependence of the proton kinetic energy in polycrystalline ice Ih between 5 K and 271 K. We compare our results with predictions form Path Integral quantum simulations and semiclassical quasi-harmonic models with phase-dependent frequencies. The latter show the best agreement with the experiment if the librational contribution is properly taken into account. The kinetic energy increase with temperature in ice is also found to be approximately a factor ∼ 5 smaller than in the case of liquid water above room temperature, highlighting the role played by anharmonic quantum fluctuations in the two phases.