A new open-shell CCSDTQ implementation and its application to the basis set convergence of post-CCSDT(Q) corrections in computational thermochemistry

We extend the CCSDTQ implementation in CFOUR to UHF and ROHF references and demonstrate its efficiency. We apply it to basis set convergence of post-CCSDT(Q) corrections for the W4-08 thermochemical dataset. Convergence of (Q)${}_{\mathrm{\Lambda }}$–(Q) is relatively rapid. For difficult species (e.g., B2, O3), CCSDTQ–CCSDT(Q)${}_{\mathrm{\Lambda }}$ may converge more slowly than (5)${}_{\mathrm{\Lambda }}$, but the effects and and basis-set trends oppose each other. Consequently, a single-shot CCCSDTQ(5)${}_{\mathrm{\Lambda }}$-CCSDT(Q)${}_{\mathrm{\Lambda }}$ correction appears most efficient. For radicals with bifurcating UHF solutions, energetics of the `less spin-contaminated’ solution are clearly more well-behaved. Our best computed adiabatic electron affinity of ozone is in excellent agreement with experiment.