2026/05/19 by Gregory H. Jones, Aditya Barman, Margarita Shepelenko +2
Physics and Astronomy · Engineering · Chemistry · #Advanced Chemical Physics Studies #Phase Equilibria and Thermodynamics #Chemical Thermodynamics and Molecular Structure
paper · pdf · doi:10.1016/j.cplett.2026.142951
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.