Due to the size of the systems in MME, we can already rule out the Dunning basis sets as being too expensive due to their higher number of primitive Gaussian-type orbitals. The def2 basis sets gave almost identical results to their minimally augmented equivalents in our previous tests of basis sets, while they were also shown to agree well with aug-cc-pVTZ/aug-cc-pVQZ results in the ROST61 benchmark study. Additionally, the improved iterative triples procedure for DLPNO-CCSD(T), typically referred to as (T 1 ), has been shown to perform better with respect to CCSD(T) for both open and closed shell species, 105,106 and therefore we consider DLPNO-CCSD(T 1 )/TightPNO/CBS(def2-TZVPP/ def2-QZVPP) our first choice level of theory for calculation of the reference values. For systems where this is too expensive, we cannot use the MP2 based composite scheme again, as MP2 is known to be unreliable for transition metal chemistry, 42,51,52,150 so we have tested three additional alternative strategies here.