Linear-Scaling Local Natural Orbital CCSD(T) Approach for Open-Shell Systems: Algorithms, Benchmarks, and Large-Scale...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties and reactivity.
• Coupled cluster (CC) methods, particularly CCSD(T), are highly accurate but computationally expensive, limiting their application to smaller systems.
• Open-shell systems, common in radicals and transition metal chemistry, present unique challenges for standard CC methods.

Purpose of the Study:

• To extend the highly optimized local natural orbital coupled cluster with single-, double-, and perturbative triple excitations (LNO-CCSD(T)) method to high-spin open-shell molecules.
• To develop an efficient and accurate computational tool for large open-shell systems.
• To enable the study of complex electronic structures in systems previously inaccessible to high-accuracy methods.

Main Methods:

• Extension of the LNO-CCSD(T) method to restricted open-shell references.
• Adoption of efficient algorithms from the closed-shell LNO-CCSD(T) variant, including iteration- and redundancy-free formulations.
• Application of integral-direct, memory-economic, and OpenMP-parallel algorithms for enhanced performance.

Main Results:

• The open-shell LNO-CCSD(T) method achieves efficiency comparable to its closed-shell counterpart for large molecules.
• Accuracy tests on radicals, ionization processes, spin-state splittings, and transition-metal compounds show average correlation energy accuracy of 99.9-99.95%.
• The method successfully modeled large systems (up to 601 atoms) with complex electronic structures, including challenging biochemical systems.

Conclusions:

• The developed open-shell LNO-CCSD(T) implementation provides a highly accurate and efficient tool for studying large and complex open-shell systems.
• The method enables accurate modeling of systems with unprecedented size and complexity using widely accessible hardware.
• This advancement opens new avenues for research in areas like catalysis, materials science, and biochemistry involving open-shell species.