where X pq encodes the rest of each term. Their trace describes the response of the correlation energy with respect to perturbations in X pq . For this reason, we can again interpret A 12 and B 12 as charge scaling terms. By contrast, terms A 5 and B 5 describe the correlation energy response to consistent changes in the one-electron energies. We want to highlight the role of A 9 , B 9 , A 13 and its omitted virtual counterpart. Rewriting the Fock matrix definition using the mean-field density ρ m f pq