"Best" Iterative Coupled-Cluster Triples Model? More Evidence for 3CC.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Electronic Structure Theory

Background:

• Coupled-cluster (CC) models are essential for accurate electronic structure calculations.
• Approximate inclusion of 3-body clusters has shown promise for improving CC model performance.
• The standard CCSD(T) model, while accurate, can be computationally expensive.

Purpose of the Study:

• To comprehensively evaluate the performance of the 3-body coupled-cluster (3CC) method for computational thermochemistry.
• To assess 3CC's accuracy against a high-level reference (CCSDTQ) within the HEAT framework.
• To investigate 3CC as a potential post-CCSD(T) method.

Main Methods:

• A new spin-integrated 3CC method was implemented, applicable to both closed- and open-shell systems.
• An automated toolchain was developed for the derivation, optimization, and evaluation of operator algebra in many-body electronic structure.
• Calculations were performed using a double-ζ basis set and extrapolated to the complete basis set limit.

Main Results:

• The 3CC method demonstrated significantly lower mean absolute errors in both electronic and atomization energies compared to CCSDT and CCSD(T) when using a double-ζ basis set.
• In the complete basis set limit, 3CC atomization energies showed a substantial error reduction compared to CCSD(T).
• The 3CC method exhibited mean absolute errors of 0.52 kJ/mol for atomization energies in the CBS limit, outperforming CCSD(T) (1.07 kJ/mol).

Conclusions:

• The 3CC method is a viable and accurate candidate for post-CCSD(T) thermochemistry applications.
• 3CC offers a more accurate and cost-effective alternative to CCSDT for general electronic structure calculations.
• The developed automated toolchain facilitates the application of 3CC to diverse chemical systems.