where X and Y are the cardinal numbers of the basis sets, and α and β are opti-mized constants specific to the basis sets used. For the def2-TZVPP/def2-QZVPP CBS (3,4) extrapolation used here, α = 7.88 and β = 2. Upon a reviewer's request, we briefly assessed the impact of spin-orbit coupling, but any such effects canceled for the REs and BHs in our tests and as such it was not necessary to include them in our reference value calculations.