As a final validation of the new JKFit auxiliary basis sets, the density fitting errors in the HF level spectroscopic constants for the diatomic molecule SO are presented in Table . SO was chosen here as the prototypical small molecule requiring additional tight-d functions in the orbital basis. Initially focusing on the DF errors with the existing cc-pVnZ/JKFit auxiliary sets, the errors in the computed spectroscopic constants are relatively small, but the errors at the QZ and 5Z level are the same, rather than decreasing for 5Z, as may be expected. An error of almost 0.1 kcal mol -1 in D e may also be unacceptably large when considering high-accuracy applications, which is a common usage for 5Z basis sets. The newly developed cc-pV(n+d)Z/JKFit auxiliary sets reduce the density fitting error at every ζ-level, there is a reduction in error as the size of the basis increases, and the errors are entirely negligible at the 5Z level. As observed in other benchmarks above, the error at the DZ level with the new auxiliary sets is only a minor improvement over cc-pVDZ/JKFit, but the BSIE at this level is large; approximately 15 kcal mol -1 for D e .