Properties of collective modes in fluids: Methodology

To interpret modern data on the dynamic structure factor for dense fluids, we have devised new and reliable methods of extracting the real and imaginary parts of the dynamic memory function of the current-current correlation function, without resorting to Laplace transforms of the experimental data. The numerical precision of these new methods is demonstrated. Also we show how our results can be used as a test of the propagation of collective modes in dense fluids, and how this relates to a recent controversy. Finally a new expression for the scattering function is suggested to improve the physical interpretation of experimental data.