where ⟨ab∥ij⟩ denotes two-electron repulsion integrals, ϵ x are orbital energies, i and j are occupied orbitals, and a and b are virtual orbitals. The error in the MP2 correlation energy due to density fitting (∆DF) is also an important metric when analysing the performance of ABSs, where ∆DF = E corr DF-MP2 -E corr MP2 . In the present case, adding an additional diffuse function for every angular symmetry occupied in the cc-pVnZ-PP-F12-MP2Fit ABS would not produce an efficient auxiliary basis as too many functions would be added. The aug-cc-pVnZ-PP-F12 orbital basis sets only add diffuse functions of higher angular symmetry, and the cc-pVnZ-PP-F12-MP2Fit ABSs of Ref. are specifically matched to orbital sets that contain diffuse s and p functions.