W4Λ: Leveraging Λ Coupled-Cluster for Accurate Computational Thermochemistry Approaches

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• High-accuracy composite wave function methods (e.g., W4 theory, HEAT, FPD) achieve sub-kJ/mol accuracy for gas-phase thermochemical properties.
• A major computational bottleneck in these methods is the evaluation of the valence post-CCSD(T) correction term.

Purpose of the Study:

• To investigate the convergence properties of the Λ coupled-cluster expansion for post-CCSD(T) corrections.
• To develop accelerated computational thermochemistry protocols without sacrificing accuracy.

Main Methods:

• Evaluation of the Λ coupled-cluster expansion, specifically CCSDT(Q)_Λ and CCSDTQ(5)_Λ.
• Application and benchmarking against the W4-17 thermochemistry dataset and its subsets.
• Comparison of convergence behavior with the regular coupled-cluster series.

Main Results:

• The Λ coupled-cluster expansion demonstrates more rapid and smoother convergence compared to the standard coupled-cluster series.
• Calculations using CCSDT(Q)_Λ and CCSDTQ(5)_Λ significantly accelerate W4- and W4.3-type computations, up to an order of magnitude.
• This acceleration is achieved without any loss in the prediction accuracy of thermochemical properties.

Conclusions:

• The developed W4Λ and W4.3Λ protocols offer a substantial speedup for high-accuracy computational thermochemistry.
• The Λ coupled-cluster expansion is an effective strategy for overcoming computational bottlenecks in accurate thermochemical calculations.
• These new protocols enhance the feasibility of high-accuracy thermochemical property predictions.