Linearized Pair-Density Functional Theory with Spin-Orbit Coupling.

Area of Science:

• Quantum Chemistry
• Computational Physics
• Theoretical Chemistry

Background:

• Pair-density functional theory (PDFT) is a method for electronic structure calculations.
• Multistate extensions of PDFT, like MC-PDFT, are used for complex systems.
• Spin-orbit coupling (SOC) is crucial for understanding electronic properties of heavy elements.

Purpose of the Study:

• To develop a new theoretical framework, SO-L-PDFT, that includes spin-orbit coupling effects.
• To address the unphysical J-symmetry breaking in existing MC-PDFT methods.
• To accurately compute electronic properties for a wide range of atoms and molecules.

Main Methods:

• Linearized pair-density functional theory (L-PDFT) was extended to include spin-orbit coupling (SOC).
• SOC integrals (1- and 2-electron) were computed using Breit-Pauli and Douglas-Kroll-Hess Hamiltonians within the atomic mean-field approximation.
• The new SO-L-PDFT method was validated against calculations of zero-field splittings, fine-structure excitation energies, and low-energy excited-state spectra.

Main Results:

• SO-L-PDFT successfully incorporates spin-orbit coupling effects into a multistate framework.
• The method eliminates the unphysical J-symmetry breaking observed in MC-PDFT.
• Accurate calculations were performed for diverse systems, including heavy atoms, ions (Ce3+, U5+), lanthanide hexachlorides, actinyl ions, and tricarbonatoactinyl complexes.
• Results were compared with spin-orbit-inclusive multireference perturbation theory.

Conclusions:

• SO-L-PDFT is a robust and accurate method for calculating electronic properties where spin-orbit coupling is significant.
• This advancement provides a reliable tool for studying systems with heavy elements.
• The method offers an improvement over existing theoretical approaches for electronic structure calculations.