Density Fitting 7,8 (DF) is a common strategy for MP2 to improve the scaling of the two-electron integral evaluations. It uses a auxiliary basis to expand the direct product of AOs, χ m (r)χ n (r) = t Q mnt ψ t (r). This allows a one order of magnitude speed increase of our code since the resulting integral evaluation only scales linearly with the basis set (or number of electrons). Specifically, the orbital potential V ijab (r) from Eq. 7 becomes