Explicitly correlated connected triple excitations in coupled-cluster theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster theory is a powerful method for electronic structure calculations.
• Incorporating explicit electron correlation is crucial for high accuracy.
• Standard coupled-cluster methods face challenges with basis set convergence for triple excitations.

Purpose of the Study:

• To develop a novel approach for integrating explicit electron correlation into connected triple excitations within coupled-cluster theory.
• To improve the accuracy and efficiency of the coupled-cluster singles and doubles model with noniterative triple excitations [CCSD(T)].

Main Methods:

• A new ansatz is proposed to incorporate explicit electron correlation.
• The ansatz is applied to the CCSD(T) model without introducing new equations.
• Implementation involves automated generation and string-based evaluation of explicit expressions.

Main Results:

• The proposed ansatz significantly enhances basis set convergence for the noniterative triple excitation correction.
• This improvement leads to better results compared to previous explicitly correlated CCSD(T) methods.
• The method does not increase the computational burden by adding new sets of equations.

Conclusions:

• The new method offers a more efficient and accurate way to include explicit electron correlation in CCSD(T) calculations.
• It effectively addresses the basis set convergence issues associated with triple excitations.
• This work represents a significant advancement in explicitly correlated coupled-cluster theory.