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\begin{equation}  G_\text{KS}(\omega) = \frac{1}{\omega - H_\text{KS}}.  \end{equation}  Thus, we find the self-energy in DFT $\Sigma_\text{KS} = V_\text{H} + V_\text{xc}$ is in fact frequency independent. In comparing with experiment, we always expect deviations at high frequencies because the non-interacting Kohn-Sham framework should only work well near the Fermi surface, where the quasiparticles of Fermi liquid theory are well-defined. However, at low frequencies (which for this section we take to mean $\omega \lesssim 1$~eV, a typical chemical scale) we may can  hope the DFT spectrum may will  resemble observations, and in many cases it does. We call these materials weakly correlated. Strongly correlated materials are those compounds where the actual self-energy $\Sigma(\omega)$ deviates strongly from the DFT reference $V_\text{H} + V_\text{xc}$ at low frequencies. We can crudely categorize the effects of correlations by considering the dominant term in the low-order expansion of the self-energy deviation  \begin{equation}