Fewest switches surface hopping with Baeck-An couplings

Area of Science:

• Computational Chemistry
• Quantum Dynamics
• Theoretical Chemistry

Background:

• The Baeck-An (BA) approximation calculates nonadiabatic coupling vectors using adiabatic energy gaps and their second derivatives.
• Accurate calculation of nonadiabatic couplings is crucial for simulating molecular dynamics involving electronic transitions.

Purpose of the Study:

• To derive and implement a time-dependent version of the Baeck-An approximation (TD-BA).
• To assess the performance of TD-BA couplings in fewest switches surface hopping simulations for molecular systems.

Main Methods:

• Derivation of the time-dependent Baeck-An (TD-BA) approximation.
• Computation of nonadiabatic couplings using energy gaps and their second time-derivatives.
• Application in fewest switches surface hopping dynamics for ethylene and fulvene.

Main Results:

• TD-BA couplings enable nonadiabatic dynamics with various electronic structure methods.
• Simulations for ethylene and fulvene show qualitatively correct dynamics and semiquantitative agreement with exact couplings.
• The method struggles with strong couplings and low velocities, and shows poor performance with TDDFT due to singlet instabilities.

Conclusions:

• TD-BA approximation provides a computationally inexpensive method for exploratory nonadiabatic dynamics.
• It is suitable for small ensembles of trajectories but requires careful consideration of its limitations.
• Further investigation is needed for its application with time-dependent density functional theory.