Second-Order Perturbative Treatment of Spin-Orbit Coupling and Ground-State Electron Correlation

Area of Science:

• Quantum Chemistry
• Computational Physics
• Relativistic Quantum Mechanics

Background:

• Accurate calculations for heavy elements require accounting for relativistic effects like spin-orbit coupling (SOC).
• Efficiently incorporating SOC into correlated wave function methods remains a computational challenge.

Purpose of the Study:

• To develop novel, efficient second-order computational approaches for treating electron correlation and SOC simultaneously.
• To provide reliable methods for electronic structure calculations of heavy-element systems.

Main Methods:

• Developed four second-order perturbative coupled-cluster methods combining electron correlation and SOC.
• Introduced SOC as a zeroth-order operator within a scalar-relativistic Hartree-Fock (SR-HF) reference.
• Compared standard perturbation theory (SOPT2) with spin-orbit coupled cluster singles (SOCCS)-based variants.

Main Results:

• SOCCS-based schemes significantly outperform SOPT2 for heavy atoms, ions, and halides.
• A CI-like SOCCS variant demonstrated the highest accuracy among the developed methods.
• Accurate spin-orbit coupling splittings were achieved using equation-of-motion coupled-cluster theory (EOM-CCSD).

Conclusions:

• The new methods offer an efficient and reliable framework for incorporating SOC in correlated electronic structure calculations.
• These approaches are particularly valuable for systems exhibiting strong relativistic effects.