  Calculating the frequency dependent dielectric function for strongly coupled
plasmas, the relations within kinetic theory and linear response theory are
derived and discussed in comparison. In this context, we proof the Kohler
variational principle for arbitrary frequencies. It can be shown to be a
special case of the Zubarev method for the construction of a non-equilibrium
statistical operator from the principle of the extremum of entropy. Within
kinetic theory, the commonly used energy dependent relaxation time approach is
strictly valid only for the Lorentz plasma in the static case. It is compared
with the result from linear response theory that includes electron-electron
interactions and applies for arbitrary frequencies, including bremsstrahlung
emission. It is shown how a general approach to linear response encompasses the
different approximations and opens options for systematic improvements.
