A diagrammatic expansion for the dynamic exchange-correlation kernel f_xc of time dependent density functional theory is formulated. It is shown that f_xc has no singularities at Kohn-Sham transition energies in every order of the perturbation theory. However, it may diverge with the system size in extended systems. This signifies that any approximate perturbative substitute for f_xc requires a consistent perturbative treatment of the equation for the response function to avoid uncontrollable errors in the many-body corrections to excitations energies.
Categories: cond-mat