MD-modeling of the intermediate scattering function for argon-like liquids and water

The spectral densities of the intermediate scattering function (ISF) for argon-like liquids and water are studied with the help of computer simulations. In the case of water, only translational motions for the centers of mass of molecules are taken into account. The applicability region for the diffusion approximation (DA) for the ISF is investigated. It is shown that the upper limit kD(T) of DA for wave vectors k→ depends on temperature and satisfies the inequality: kD(T) < < 1/a, where a is the interparticle spacing. Analogously, the upper limit ωDk→T for frequencies depends on wave vector and temperature and it is surprisingly small: ωDk→T~γDk→2, where γDk→2 is the half-width for the Lorentzian: γDk→2~Dsk→2, and Ds is the self-diffusion coefficient. The possibility to estimate the ratio Dc/Ds, where Dc is the collective part of the self-diffusion coefficient, is discussed. It has been shown that k→-dependencies of the half-width γDk→2 for water and argon spectra are similar to each other. It means that the behavior of γDk→2 is mainly determined by translational motion of molecules.