W4$\mathrm{\Lambda }$: leveraging $\mathrm{\Lambda }$ coupled cluster for accurate computational thermochemistry approaches

High-accuracy composite wavefunction methods like Weizmann-4 (W4) theory, high-accuracy extrapolated $\mathit{\text{ab}}\mathit{\text{initio}}$ thermochemistry (HEAT), and Feller-Peterson-Dixon (FPD) 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 lambda coupled cluster expansion converges more rapidly and smoothly than the regular coupled cluster series. By means of CCSDT(Q)${}_{\mathrm{\Lambda }}$ and CCSDTQ(5)${}_{\mathrm{\Lambda }}$, we can considerably (up to an order of magnitude) accelerate W4- and W4.3-type calculations without loss in accuracy, leading to the W4$\mathrm{\Lambda }$ and W4.3$\mathrm{\Lambda }$ computational thermochemistry protocols.