We present a new paradigm for modeling electronically nonadiabatic molecular dynamics by extending the specific reaction parameter (SRP) approach to active space configuration interaction with single, double, and triple excitations in order to model coupled potential energy surfaces. Using the photoisomerization of 1,3-cyclohexadiene (CHD) as an example application, we develop ODM3.25(CHD), a system-specific reparameterization of the semiempirical ODM3.25 method trained against XMS-CASPT2 energies for the S0, S1, and S2 states at 1001 geometries. The resulting model achieves a mean unsigned error of 0.195 eV─comparable to the intrinsic accuracy limits of multireference perturbation theory─while automatically preserving (without diabatization) the correct topology of conical intersections through its configuration-interaction framework. Coupled with curvature-driven coherent switching with decay of mixing (κCSDM), ODM3.25(CHD) enables large-ensemble, 800 fs nonadiabatic trajectory simulations that reproduce key experimental and theoretical features of CHD photochemistry, including excited-state lifetimes of 60-70 fs, early passage through S1/S2 intersections, and ring-opening quantum yields of 43-53%. ODM3.25-SRP enables nonadiabatic dynamics of cyclohexadiene with XMS-CASPT2 quality and an expanded active space while reducing the computation time by a factor of 65. This work demonstrates that treating ODM3.25 as a domain-specific foundation model and fine-tuning it via SRP yields a computationally efficient, chemically accurate multistate potential capable of describing complex photodynamics at a fraction of the cost of high-level ab initio methods. This establishes SRP-based custom excited-state models as a powerful new tool for large-scale nonadiabatic dynamics.
Shu et al. (Wed,) studied this question.