@SusiLehtola Thanks for pointing this out. I checked the AutoAux paper (DOI: 10.1021/acs.jctc.6b01041), and it seems that for double zeta basis sets, AutoAux is usually more accurate than hand-tuned aux basis sets for total energies, but less accurate for reaction energies. In the most unfavorable case (MP2 reaction energies of the DC9 benchmark set, Table 5), the average error of AutoAux is 0.16 kcal/mol, while that of the hand-tuned basis sets (def2/JK + def2/C) is 0.06 kcal/mol. I'm not sure if this suffices to show that AutoAux is unsuitable for double zeta basis sets. —
wzkchem5 Sep 27 at 15:45