FNO-CCSDTQ(5)Λ as an economical alternative for connected quintuple excitations contributions in coupled cluster thermochemistry

Contributions from connected quintuple excitations in coupled cluster theory can reach the 0.5 kcal mol − 1 range, important enough to matter in accurate computational thermochemistry, yet the very steep ∝ N 12 CPU time scaling impedes routine evaluation. We show that for the differential contribution of quintuples, convergence of a frozen natural orbital (FNO) expansion with respect to the NO cutoff is rapid enough to make FNO-CCSDTQ(5) Λ with cutoffs of 0.0025 or 0.001 viable alternatives. A naive extrapolation to zero cutoff from {0.005,0.0025} works surprisingly well as a low-cost option. Interestingly, FNO convergence is definitely slower for second-row than for first-row compounds.