New Framework for Multi-Electronic-State Dynamics and Its Validation for Photoisomerization of 1,3-Cyclohexadiene

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Photochemistry

Background:

• Modeling nonadiabatic molecular dynamics is computationally demanding.
• Accurate simulation of coupled potential energy surfaces is crucial for understanding photochemical reactions.

Purpose of the Study:

• To develop a new paradigm for modeling electronically nonadiabatic molecular dynamics.
• To extend the specific reaction parameter (SRP) approach to active space configuration interaction.
• To enable accurate and efficient simulations of complex photodynamics.

Main Methods:

• Extended the specific reaction parameter (SRP) approach to active space configuration interaction with single, double, and triple excitations.
• Developed ODM3.25(CHD), a system-specific reparameterization trained against XMS-CASPT2 energies.
• Employed curvature-driven coherent switching with decay of mixing (κCSDM) for trajectory simulations.

Main Results:

• Achieved a mean unsigned error of 0.195 eV, comparable to multireference perturbation theory.
• Successfully reproduced key experimental and theoretical features of 1,3-cyclohexadiene (CHD) photochemistry.
• Reduced computation time by a factor of 65 compared to high-level ab initio methods.

Conclusions:

• SRP-based custom excited-state models provide a computationally efficient and chemically accurate approach for large-scale nonadiabatic dynamics.
• This method establishes a powerful new tool for studying complex photochemical processes.
• The developed ODM3.25(CHD) model accurately describes the photodynamics of cyclohexadiene.