SOiCI and iCISO: combining iterative configuration interaction with spin-orbit coupling in two ways

Area of Science:

• Quantum chemistry
• Relativistic quantum mechanics

Background:

• The iCIPT2 method accurately treats static and dynamic electron correlation.
• Relativistic effects are crucial for describing heavy elements.

Purpose of the Study:

• Extend the iCIPT2 approach to the relativistic domain.
• Develop accurate and efficient methods for calculating spin-orbit coupling (SOC).

Main Methods:

• Incorporated scalar relativity using the spin-free X2C Hamiltonian.
• Treated spin-orbit coupling (SOC) with a Douglas-Kroll-Hess-like operator.
• Developed two relativistic iCIPT2 variants: SOiCI and iCISO.
• Utilized double group and time reversal symmetries for computational efficiency.

Main Results:

• SOiCI demonstrated high accuracy for the spin-orbit splitting (SOS) of heavy atoms.
• iCISO proved computationally efficient for SOS calculations in light atoms.
• iCISO is suitable for systems with quenched SOC, including those with heavy atoms.

Conclusions:

• The relativistic iCIPT2 framework provides accurate and efficient methods for electronic structure calculations.
• SOiCI and iCISO offer complementary approaches for studying relativistic effects in various chemical systems.