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

Abstract

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)${}_{\Lambda }$--(Q) is relatively rapid. For difficult species (e.g., B2, O3), CCSDTQ--CCSDT(Q)${}_{\Lambda }$ may converge more slowly than (5)${}_{\Lambda }$, but the effects and and basis-set trends oppose each other. Consequently, a single-shot CCCSDTQ(5)${}_{\Lambda }$-CCSDT(Q)${}_{\Lambda }$ correction appears most efficient. For radicals with bifurcating UHF solutions, energetics of the `less spin-contaminated' solution are clearly more well-behaved. Alternatives to a single-shot CCSDTQ(5)${}_{\Lambda }$--CCSDT(Q)${}_{\Lambda }$ correction are evaluating (5)${}_{\Lambda }$ 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.

Final published version CC:BY 4.0

References in corpus (1)

• FNO-CCSDTQ(5)${}_{\hat{I}}›$ as an economical alternative for connected quintuple excitations contributions in coupled cluster thermochemistry