Comparison of Spin-Flip TDDFT-Based Conical Intersection Approaches with XMS-CASPT2

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Quantum Chemistry

Background:

• Conical intersection geometries are vital for understanding molecular photochemical reactivity.
• Accurate methods like multireference approaches are computationally expensive for large molecules.
• Developing efficient computational methods is essential for studying larger systems.

Purpose of the Study:

• To evaluate the accuracy of spin-flip time-dependent density functional theory (SF-TDDFT) for determining minimum energy crossing point (MECP) conical intersection geometries.
• To compare SF-TDDFT results with higher-level methods such as XMS-CASPT2 and CASSCF.
• To assess the computational cost and performance of SF-TDDFT with and without explicit nonadiabatic coupling terms.

Main Methods:

• Spin-flip time-dependent density functional theory (SF-TDDFT) within the Tamm-Dancoff Approximation.
• Calculation of MECP geometries with and without explicit nonadiabatic coupling terms.
• Comparison with geometries obtained from XMS-CASPT2 and CASSCF methods.

Main Results:

• SF-TDDFT methods without explicit nonadiabatic coupling terms generally agree well with XMS-CASPT2 reference structures.
• Relative energetics are reasonably reproduced when using the BHHLYP functional and full nonadiabatic coupling terms for MECP structures.
• CASSCF calculations occasionally show quantitative deviations from XMS-CASPT2, highlighting the need for dynamical correlation.

Conclusions:

• SF-TDDFT presents a computationally feasible approach for determining conical intersection geometries in larger molecules.
• The accuracy of SF-TDDFT depends on the inclusion of nonadiabatic coupling terms and the chosen functional.
• Dynamical correlation effects are important for accurate geometry determination, as shown by CASSCF deviations.