Spin-Adapted Restricted Open-Shell Hartree-Fock and Its Dynamic Correlation Extension.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• Restricted open-shell Hartree-Fock (ROHF) methods offer a balance between accuracy and computational cost.
• Challenges remain in optimizing electronic configurations and capturing dynamic correlation effects.

Purpose of the Study:

• To implement and evaluate a spin-adapted configuration-state-function restricted open-shell Hartree-Fock (CSF-ROHF) method in OpenMolcas.
• To assess the computational efficiency and convergence behavior of the CSF-ROHF approach.
• To develop and validate a computationally inexpensive protocol for incorporating dynamic correlation to improve CSF-ROHF accuracy.

Main Methods:

• Development of a spin-adapted CSF-ROHF implementation using the Graphical Unitary Group Approach and Generalized Active Space algorithms.
• Analysis of CSF-ROHF convergence properties, emphasizing the role of initial orbital ordering.
• Introduction of a second-order, spin-adapted perturbation strategy with the Stochastic-SplitGAS algorithm to include dynamic correlation.

Main Results:

• The CSF-ROHF method enables orbital optimization of single electronic configurations at near mean-field cost.
• Computational efficiency was demonstrated on {[Ni(H2O)4]nOn-1(H2O)2}2+ model systems.
• CSF-ROHF showed limitations for iron-sulfur cluster spin gaps, but perturbation theory correction yielded excellent agreement with high-level methods.

Conclusions:

• The developed CSF-ROHF method provides an efficient route for electronic structure calculations.
• Initial orbital ordering is critical for successful CSF-ROHF optimization.
• Perturbative correction significantly enhances the accuracy of CSF-ROHF, offering a cost-effective approach for complex systems.