The third alternative level of theory, for systems where even DLPNO-CCSD/TightPNO/def2-QZVPP calculations were unfeasible, uses the PNO extrapolation scheme described by This involves calculating results with increasing values of the threshold T CutP N O , which defines the PNOs included in the final Coupled Cluster treatment, while maintaining all other cutoffs at the same values. To approximate standard TightPNO results, where T CutP N O = 10 -7 , DLPNO-CCSD(T 1 ) calculations are done with the looser T CutP N O values of 10 -5 and 10 -6 , and the correlation energies are extrapolated for each basis set as such: