Prospects for rank-reduced CCSD(T) in the context of high-accuracy thermochemistry

TL;DR

This paper explores rank-reduced approximations to the (T) component in CCSD(T) to enable high-accuracy thermochemical calculations on medium-sized molecules with reduced computational cost.

Contribution

It introduces and assesses new approximate methods for the (T) correction, demonstrating their accuracy and efficiency in high-accuracy thermochemistry calculations.

Findings

All tested approximations achieve sub-0.1 kJ/mol accuracy.

The $ ilde{Z}T$ approximation offers the best cost-accuracy balance.

Order-of-magnitude reduction in CCSD(T) computational cost is possible.

Abstract

Obtaining sub-chemical accuracy (1 kJ mol${}^{-1}$) for reaction energies of medium-sized gas-phase molecules is a longstanding challenge in the field of thermochemical modeling. The perturbative triples correction to CCSD, CCSD(T), constitutes an important component of all high-accuracy composite model chemistries that obtain this accuracy, but can be a roadblock in the calculation of medium to large systems due to its $\mathcal{O}(N^7)$ scaling, particularly in HEAT-like model chemistries that eschew separation of core and valance correlation. This study extends the work of Lesiuk [J. Chem. Phys. 156, 064103 (2022)] with new approximate methods and assesses the accuracy of five different approximations of (T) in the context of a subset of molecules selected from the W4-17 dataset. It is demonstrated that all of these approximate methods can achieve sub-0.1 kJ mol${}^{-1}$ accuracy with respect to canonical, density-fitted (T) contributions with a modest number of projectors. The approximation labeled $\tilde{Z}T$ appears to offer the best trade-off between cost and accuracy and shows significant promise in an order-of-magnitude reduction in the computational cost of the CCSD(T) component of high-accuracy model chemistries.