  Warm dense matter (WDM) has emerged as one of the frontiers of both
experimental and theoretical physics and is challenging traditional concepts of
plasma, atomic, and condensed-matter physics. While it has become common
practice to model correlated electrons in WDM within the framework of Kohn-Sham
density functional theory, quantitative benchmarks of exchange-correlation (XC)
functionals under WDM conditions are yet incomplete. Here, we present the first
assessment of common XC functionals against exact path-integral Monte Carlo
calculations of the harmonically perturbed thermal electron gas. This system is
directly related to the numerical modeling of X-Ray scattering experiments on
warm dense samples. Our assessment yields the parameter space where common XC
functionals are applicable. More importantly, we pinpoint where the tested XC
functionals fail when perturbations on the electronic structure are imposed. We
indicate the lack of XC functionals that take into account the needs of WDM
physics in terms of perturbed electronic structures.
