TD-DFT spin-adiabats with analytic nonadiabatic derivative couplings

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Nonadiabatic derivative couplings are crucial for understanding conical intersections and photochemical reactions.
• Efficient calculation methods are needed for complex molecular systems.

Purpose of the Study:

• To develop an efficient algorithm for analytic nonadiabatic derivative couplings.
• To implement and validate the algorithm within time-dependent density functional theory (TD-DFT) and the Tamm-Dancoff approximation (TDA).

Main Methods:

• Direct differentiation of Kohn-Sham pseudowavefunctions.
• Implementation for closed-shell ground states with an even number of electrons.
• Validation against finite difference methods at a benzaldehyde S₁/T₂ crossing.
• Introduction of a magnetic field spin-coupling operator to break time-reversal symmetry.

Main Results:

• The algorithm efficiently calculates analytic nonadiabatic derivative couplings.
• Complex-valued couplings were generated by breaking time-reversal symmetry.
• A phase rotation successfully transformed complex couplings into a nearly real-valued vector for benzaldehyde.

Conclusions:

• The presented algorithm offers an efficient route to compute nonadiabatic derivative couplings.
• The method provides insights into spin-related phenomena in molecular systems.
• The ability to obtain real-valued couplings simplifies their application in dynamics simulations.