A large-scale molecular dynamics study of dynamic structure factor and dispersion relation of acoustic mode in liquid and supercritical water

Large-scale and long-time molecular dynamics calculations have been performed using a fluctuating charge model TIP4P-FQ in order to investigate structure and dynamics of supercritical water near the critical point. The calculated Ornstein–Zernike behavior of the structure factor S ( k ) in the small wave number region was in satisfactory agreement with experiment. The intermediate scattering function F ( k , t ) showed much slower decay in the supercritical water than that found in the ambient water. Further, the calculated dynamic structure factor S ( k , ω ) clearly showed the existence of acoustic mode for both systems. The calculated dispersion relation for the liquid water is in good agreement with the IXS data. In the supercritical water near the critical point, the acoustic excitation propagates with a speed of about 420 m/s.