deletions | additions       diff --git a/correlations.tex b/correlations.tex  index f57c032..9157f80 100644  --- a/correlations.tex  +++ b/correlations.tex 
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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 $\Sigma_\text{KS} = V_\text{xc}$ 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 can hope the DFT spectrum will resemble observations, and in many cases it does. We call these materials weakly correlated. Strongly correlated materials are those compounds where the difference between  actual self-energy $\Sigma(\omega)$ deviates strongly from the and  DFT reference $V_\text{xc}$ $\left|\Sigma(\omega) - \Sigma_\text{KS}\right|$ is large  at low frequencies. We note that defining correlations by measuring how much $\Sigma$ deviates from the reference provided by DFT is not the only route. If we were chemists, rather than solid state physicists, we would use the self-energy produced by the Hartree-Fock approximation $\Sigma_\text{HF}$ as our reference. Returning to solid state point of view, 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}