W4Λ: Leveraging Λ Coupled-Cluster for Accurate Computational Thermochemistry Approaches

High-accuracy composite
wave function methods like Weizmann-4 (W4)
theory, high-accuracy extrapolated ab initio thermochemistry (HEAT),
and the Feller–Peterson–Dixon (FPD) approach enable
sub-kJ/mol accuracy in gas-phase thermochemical properties. Their
biggest computational bottleneck is the evaluation of the valence
post-CCSD(T) correction term. We demonstrate here, for the W4-17 thermochemistry
benchmark and subsets thereof, that the Λ coupled-cluster expansion
converges more rapidly and smoothly than the regular coupled-cluster
series. By means of CCSDT(Q)Λ and CCSDTQ(5)Λ, we can considerably (up to an order of magnitude) accelerate W4-
and W4.3-type calculations without loss in accuracy, leading to the
W4Λ and W4.3Λ computational thermochemistry protocols.