"Best" iterative coupled-cluster triples model: More evidence for 3CC

TL;DR

The paper proposes and implements the 3CC iterative triples model as an accurate, cost-efficient alternative to CCSDT for post-CCSD(T) thermochemistry. By integrating a spin-integrated, automated toolchain (SeQuant, TiledArray, MPQC), the authors assess 3CC on the HEAT benchmark for both closed- and open-shell systems, comparing against CCSDT, CCSD(T), and higher-order variants. Key findings show that 3CC delivers markedly better valence correlation and atomization energies than CCSD(T) and CCSDT, and in the CBS limit achieves errors competitive with or superior to higher-cost methods, suggesting 3CC as the recommended iterative triples heuristic and a practical starting point for quadruple corrections. The work also outlines a scalable implementation framework and discusses avenues to reduce cost further through tensor factorization and sparsification, broadening the applicability of high-accuracy CC methods to larger systems.

Abstract

To follow up on the unexpectedly-good performance of several coupled-cluster models with approximate inclusion of 3-body clusters [J. Chem. Phys. 151, 064102 (2019)] we performed a more complete assessment of the 3CC method [J. Chem. Phys. 125, 204105 (2006)] for accurate computational thermochemistry in the standard HEAT framework. New spin-integrated implementation of the 3CC method applicable to closed- and open-shell systems utilizes a new automated toolchain for derivation, optimization, and evaluation of operator algebra in many-body electronic structure. We found that with a double-zeta basis set the 3CC correlation energies and their atomization energy contributions are almost always more accurate (with respect to the CCSDTQ reference) than the CCSDT model as well as the standard CCSD(T) model. The mean absolute errors in cc-pVDZ {3CC, CCSDT, and CCSD(T)} electronic (per valence electron) and atomization energies relative to the CCSDTQ reference for the HEAT dataset [J. Chem. Phys. 121, 11599 (2004)], were {24, 70, 122} $μE_h/e$ and {0.46, 2.00, 2.58} kJ/mol, respectively. The mean absolute errors in the complete-basis-set limit {3CC, CCSDT, and CCSD(T)} atomization energies relative to the HEAT model reference, were {0.52, 2.00, and 1.07} kJ/mol, The significant and systematic reduction of the error by the 3CC method and its lower cost than CCSDT suggests it as a viable candidate for post-CCSD(T) thermochemistry applications, as well as the preferred alternative to CCSDT in general.