For determining the reference method for our geometry benchmark we employed Molpro, as analytical gradients are available for DF-MP2 52 /aug-cc-pVnZ (n = D, T, Q), DF-MP2-F12 /cc-pVnZ-F12 (n = D, T), and DF-CCSD(T)-F12b 56,57 /cc-pVDZ-F12. In all cases the default parameters, or as specified by their documentation, was used a . We used DF-CCSD(T)-F12b/cc-pVDZ-F12 as the highest level of theory to determine which variant of MP2 to use on the larger test set, as discussed in Section 2.1.1 and Section 3.1.