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) Λ –(Q) is relatively rapid. For difficult species (e.g., B 2 , O 3 ), CCSDTQ–CCSDT(Q) Λ may converge more slowly than (5) Λ , but the effects and and basis-set trends oppose each other. Alternatives to a single-shot CCSDTQ(5) Λ –CCSDT(Q) Λ correction are evaluating (5) Λ either in a truncated cc-pVDZ(p,s) basis set, or by means of frozen natural orbitals. Our best computed adiabatic electron affinity of ozone is in excellent agreement with experiment.