Scalable Electron Correlation Methods. 7. Local Open-Shell Coupled-Cluster Methods Using Pair Natural Orbitals:...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Method Development

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• High-spin open-shell systems present unique challenges in quantum chemical methods.
• Existing coupled cluster methods can be computationally expensive for large molecules.

Purpose of the Study:

• To develop and implement well-parallelized, local coupled cluster methods for high-spin open-shell systems.
• To introduce efficient approximations using pair natural orbitals (PNOs) for reduced computational cost.
• To enable accurate and feasible calculations of large open-shell molecules.

Main Methods:

• Implementation of spin-orbital coupled cluster theory with restricted open-shell Hartree-Fock (ROHF) reference functions.
• Development of two variants: PNO-UCCSD (spin-unrestricted) and PNO-RCCSD (spin-adapted).
• Application of domain and pair approximations for near linear scaling, utilizing projected atomic orbitals (PAOs).

Main Results:

• Achieved near linear scaling computational cost with respect to the number of correlated electrons.
• Demonstrated convergence of results with respect to approximation thresholds.
• Reduced errors in relative energies to below 1 kcal mol⁻¹ for challenging cases.
• Enabled calculations for molecules with 100-200 atoms using augmented triple-ζ basis sets in hours on ~100 CPU cores.

Conclusions:

• The developed PNO-UCCSD and PNO-RCCSD methods provide a computationally efficient and accurate approach for high-spin open-shell systems.
• These methods are a significant step towards fully local open-shell coupled cluster (T) methods.
• The implementation allows for accurate electronic structure calculations of large, complex open-shell molecules.