Efficient and accurate local single reference correlation methods for high-spin open-shell molecules using pair...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Previous work established closed-shell Local Pair Natural Orbital (LPNO) methods.
• High-spin open-shell calculations present unique technical challenges.

Purpose of the Study:

• To report a production-level implementation of high-spin open-shell coupled cluster methods using the LPNO concept.
• To extend the successful closed-shell LPNO methods to open-shell systems.
• To detail technical solutions for open-shell calculations.

Main Methods:

• Implementation of coupled pair, quadratic configuration interaction, and coupled cluster methods with up to doubly excited determinants.
• Utilized the Local Pair Natural Orbital (LPNO) framework.
• Employed density fitting (resolution of the identity) techniques for integral transformations.
• Used quasi-restricted orbitals for the reference determinant construction.

Main Results:

• The LPNO approach was successfully extended to high-spin open-shell systems.
• Numerical tests confirmed high accuracy, reproducing parent canonical method results.
• The method demonstrated outstanding performance and efficiency, similar to closed-shell counterparts.
• Hyperfine couplings calculated with LPNO-CEPA/1 showed the method's potential for property calculations.

Conclusions:

• The developed LPNO methods offer an efficient and accurate approach for high-spin open-shell electronic structure calculations.
• The implementation is robust, controlled by three cutoff parameters, and achieves high accuracy.
• The LPNO approach holds significant promise for the efficient computation of molecular properties, including hyperfine couplings.