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Rigid molecule approximation in memory function-based models for molecular liquids: application to liquid water
In the present article we show how models for simple liquids can be used to describe the dynamics of atoms in molecular liquids within the rigid molecule approximation. We show in particular that the atomic masses are to be replaced by the corresponding Sachs-Teller masses and we derive a formal expression for the so-called Einstein frequency. The approach is illustrated for a model which has been originally developed for simple liquids and which has been used in the past to analyze quasielastic neutron scattering data from pure water and dilute aqueous solutions of apolar molecules. We obtain a remarkable agreement with results from molecular dynamics simulations not only in the quasielastic, diffusive regime, but also in the inelastic regime corresponding to intermolecular vibrations and fast molecular librations.