Approximately size extensive local Multireference Singles and Doubles Configuration Interaction.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multi-reference Configuration Interaction (MRCI) is a highly accurate method for small molecules.
• Traditional MRCI methods have high computational costs and are not size-extensive, limiting their application to larger systems.
• Existing methods struggle with accurate electronic structure predictions for molecules beyond a certain size.

Purpose of the Study:

• To develop a computationally efficient and size-extensive MRCI method applicable to larger molecules.
• To address the limitations of traditional MRCI in terms of computational scaling and energy errors.
• To enable accurate electronic structure and reaction energetics predictions for medium-to-large molecules.

Main Methods:

• Developed a local (L) MRCI approach combined with Cholesky decomposition (CD) for efficient integral processing.
• Implemented integral screening and truncation of long-range electron correlation in a local orbital basis.
• Investigated a priori and a posteriori size extensivity corrections, including Multi-reference Averaged Coupled-Pair Functional (MRACPF) and Davidson-Silver/Pople schemes.

Main Results:

• Achieved a computational cost scaling of O(N(3)) with a small prefactor, significantly reducing computational expense.
• Demonstrated that CD-LMRACPF provides superior accuracy compared to Davidson-type corrections.
• Successfully applied the CD-LMRACPF method to molecules with up to 50 heavy atoms.

Conclusions:

• The developed CD-LMRACPF method offers a computationally feasible and accurate approach for electronic structure calculations of larger molecules.
• This method overcomes the limitations of traditional MRCI, enabling high-accuracy predictions for systems previously intractable.
• The advancements pave the way for more sophisticated theoretical studies in chemistry and materials science involving larger molecular systems.