Single-particle dynamics in simple liquids

Despite its apparent simplicity, a comprehensive interpretation of all the features of the motion of a tagged particle in a liquid is a challenging task. The basic reason for this difficulty lies in the wealth of physical events spanning vastly different length- and time-scales (such as streaming, collisions, cage effects, diffusion) which must be accounted for. In this respect, the accurate data presently available from both incoherent neutron scattering and computer simulation also pose a rather severe benchmark for the theory. In this contribution we shall attempt to encompass all the aspects of the problem by a unified theoretical framework in which phenomenology is kept to a minimum. To consider a specific case, the results are compared with the data in liquid sodium near the melting point, obtained by inelastic neutron scattering experiments, as well as by new, extensive simulations performed on purpose for such an investigation.