Dynamic structure factor and collective excitations of neutral and Coulomb fluids

The dynamic sructure factor as the basic quantity describing the collective excitations in a fluid is considered. We consider the cases of neutral and Coulombic fluids. The classical method of moments is applied to construct the dynamic structure factor satisfying all known sum rules. An interpolational formula is found which expresses the dynamic characteristics of a classical or quantum fluid system in terms of its static correlation parameters. The analytical results based on the theory of moments are compared with Molecular dynamics data for various model systems.