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

TL;DR

This paper introduces an efficient implementation of open-shell CCSDTQ methods in CFOUR, demonstrating rapid basis set convergence and applying it to thermochemical data, with improved accuracy for challenging species.

Contribution

The authors extend CCSDTQ implementation to UHF and ROHF references and analyze its efficiency and convergence in thermochemical calculations.

Findings

Convergence of post-CCSDT(Q) corrections is rapid for most species.

Single-shot CCCSDTQ(5)$_ extLambda$-CCSDT(Q)$_ extLambda$ correction is most efficient.

Best computed ozone electron affinity matches experimental data.

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. Our best computed adiabatic electron affinity of ozone is in excellent agreement with experiment.