Collective modes in simple liquids: A semiempirical model

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Abstract
A semiempirical model for the dynamical structure factor, S(Q,ω), in a simple liquid is presented. This is a modification of the Lorentzian form predicted by linear hydrodynamics that enable it to be extended into the viscoelastic region while still retaining finite moments. The model leads directly to a criterion for the existence of a sound mode which includes the possibility of soft modes and dispersion gaps. By a suitable choice of parameters the experimental results for liquid Ar and molten Rb may be explained.