Machine Learning the Nonradiative Decay Modes in Photochemical Processes

Nonradiative decay in photoexcited molecular systems is driven by nuclear motion toward conical intersections (CIs), where electronic states become degenerate and nonadiabatic transitions occur. Identifying the nuclear degrees of freedom responsible for CI access from nonadiabatic molecular dynamics (NAMD) simulations remains challenging, because the underlying motions are high-dimensional and collective. Here, we introduce an unsupervised, information-theoretic framework based on differentiable information imbalance (DII) to identify the nuclear coordinates governing CI access directly from trajectory surface hopping (TSH) simulations. By quantifying correlations between structural descriptors and electronic observables, including energy gaps and oscillator strengths, the method ranks nuclear degrees of freedom by their predictive relevance. A multistep protocol then extracts low-dimensional, physically interpretable modes associated with nonradiative decay. We apply the framework to the methaniminium cation, furan, l-glutamine, l-pyroglutamine ammonium, and a photoactive molecular motor. Across all systems, the method recovers known mechanistic coordinates while revealing the relative importance of competing modes when multiple structural distortions contribute to the CI access. The analysis also reveals a systematic distinction between observables: energy gaps are typically governed by a small number of localized coordinates, whereas oscillator strengths depend on more-extended and distributed structural rearrangements. Overall, the DII-based framework combines predictive power with direct interpretability, providing a general and scalable route for extracting mechanistic insights from high-dimensional NAMD data and constructing reduced-dimensional models of excited-state dynamics.