Conical Intersections in Organic Molecules: Benchmarking Mixed-Reference Spin-Flip Time-Dependent DFT (MRSF-TD-DFT)...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Spin-flip time-dependent density functional theory (SF-TD-DFT) suffers from spin contamination.
• Existing methods struggle to accurately describe conical intersections in organic molecules.

Purpose of the Study:

• Introduce and validate the mixed-reference spin-flip time-dependent density functional theory (MRSF-TD-DFT) method.
• Enable automatic geometry optimization of targeted states, specifically S1/S0 conical intersections.
• Compare MRSF-TD-DFT accuracy with established ab initio methods.

Main Methods:

• Application of the MRSF-TD-DFT method for geometry optimization.
• Calculation of analytic gradients for MRSF-TD-DFT response states.
• Optimization of S1/S0 conical intersections in typical organic molecules.

Main Results:

• MRSF-TD-DFT successfully produces the correct double-cone topology of conical intersections.
• The method accurately describes the geometry and relative energies of lowest-energy conical intersections.
• MRSF-TD-DFT achieves accuracy comparable to high-level multireference wavefunction ab initio methods.

Conclusions:

• MRSF-TD-DFT overcomes spin contamination issues present in SF-TD-DFT.
• The method enables reliable automatic geometry optimization and molecular dynamics simulations.
• MRSF-TD-DFT offers a robust alternative to single-reference methods for describing conical intersections.