Assessment of the accuracy of coupled cluster perturbation theory for open-shell systems. I. Triples expansions

Abstract

The accuracy at which total energies of open-shell atoms and organic radicals may be calculated is assessed for selected coupled cluster perturbative triples expansions, all of which augment the coupled cluster singles and doubles (CCSD) energy by a non-iterative correction for the effect of triple excitations. Namely, the second- through sixth-order models of the recently proposed CCSD(T-n) triples series [J. Chem. Phys. 140, 064108 (2014)] are compared to the acclaimed CCSD(T) model for both unrestricted as well as restricted open-shell Hartree-Fock (UHF/ROHF) reference determinants. By comparing UHF- and ROHF-based statistical results for a test set of 18 modest-sized open-shell species with comparable RHF-based results, no behavioral differences are observed for the higher-order models of the CCSD(T-n) series in their correlated descriptions of closed- and open-shell species. In particular, we find that the convergence rate throughout the series towards the coupled cluster singles, doubles, and triples (CCSDT) solution is identical for the two cases. For the CCSD(T) model, on the other hand, not only its numerical consistency, but also its established, yet fortuitous cancellation of errors breaks down in the transition from closed- to open-shell systems. The higher-order CCSD(T-n) models (orders n>3) thus offer a consistent and significant improvement in accuracy relative to CCSDT over the CCSD(T) model, equally for RHF, UHF, and ROHF reference determinants, albeit at an increased computational cost.

22 pages, 5 figures, 1 table, 1 supporting information (attached as an ancillary file)

References in corpus (4)

• W4 theory for computational thermochemistry: in pursuit of confident sub-kJ/mol predictions
• Assessment of the accuracy of coupled cluster perturbation theory for open-shell systems. I. Triples expansions
• A view on coupled cluster perturbation theory using a bivariational Lagrangian formulation
• Assessment of the accuracy of coupled cluster perturbation theory for open-shell systems. II. Quadruples expansions

Cited by in corpus (9)

• The Variational Quantum Eigensolver: a review of methods and best practices
• Calculation of Ligand Dissociation Energies in Large Transition-Metal Complexes
• Many-Body Expanded Full Configuration Interaction. I. Weakly Correlated Regime
• Many-Body Expanded Full Configuration Interaction. II. Strongly Correlated Regime
• Assessment of the accuracy of coupled cluster perturbation theory for open-shell systems. I. Triples expansions
• Assessment of the accuracy of coupled cluster perturbation theory for open-shell systems. II. Quadruples expansions
• Convergence of coupled cluster perturbation theory
• Factorized Quadruples and a Predictor of Higher-Level Correlation in Thermochemistry
• Limitations of coupled cluster approximations for highlyaccurate investigations of Rb${}_2^{+}$