Accurate Spin-Orbit Coupling by Relativistic Mixed-Reference Spin-Flip-TDDFT.

Area of Science:

• Quantum Chemistry
• Relativistic Electronic Structure Theory

Background:

• Spin-orbit coupling (SOC) is crucial for understanding electronic transitions in molecules.
• Accurate relativistic calculations are essential for heavy elements and processes like intersystem crossing.

Purpose of the Study:

• To develop and validate a relativistic mixed-reference spin-flip time-dependent density functional theory (SOC-MRSF-TDDFT) method.
• To assess the accuracy of SOC-MRSF-TDDFT for spin-orbit energy splitting and intersystem crossing predictions.

Main Methods:

• Development of SOC-MRSF-TDDFT within the mean-field approximation.
• Comparison with experimental data and other high-level relativistic methods (4c-CASSCF, 4c-CASPT2, SO-GMC-QDPT2).
• Application to atomic systems (C, Si, Ge, Sn) and molecular systems (4-thiothymine, thymine).

Main Results:

• SOC-MRSF-TDDFT accurately reproduces experimental spin-orbit energy splittings for atoms C, Si, Ge, and Sn.
• Calculations for 4-thiothymine show excellent agreement with SO-GMC-QDPT2.
• The method successfully predicted intersystem crossing (S1 → T1) in thymine.

Conclusions:

• SOC-MRSF-TDDFT is a highly accurate and practical computational protocol.
• It is well-suited for challenging electronic structure problems, including nonadiabatic molecular dynamics (NAMD).
• The method facilitates the study of internal conversions and intersystem crossings in large molecular systems.