  Fundamental properties of warm dense matter are described by the dielectric
function, which gives access to the frequency-dependent electrical
conductivity, absorption, emission and scattering of radiation, charged
particles stopping and further macroscopic properties. Different approaches to
the dielectric function and the related dynamical collision frequency are
compared in a wide frequency range. The high-frequency limit describing inverse
bremsstrahlung and the low-frequency limit of the dc conductivity are
considered. Sum rules and Kramers-Kronig relation are checked for the
generalized linear response theory and the standard approach following kinetic
theory. The results are discussed in application to aluminum, xenon and argon
plasmas.
