Advances in Excited-State Dynamics: Bridging Small Molecules and Large Systems

Non-adiabatic dynamics govern processes in which the Born–Oppenheimer approximation fails, allowing for strong coupling between electronic and nuclear degrees of freedom. These effects are crucial in photochemistry, where electronic transitions occur on ultrafast timescales. Near conical intersections, the interplay between potential energy surfaces enables efficient radiationless decay and determines reaction pathways, influencing molecular stability and quantum yields.

Recent advances in mixed quantum-classical and fully quantum approaches, such as surface hopping and multi-configurational time-dependent methods, have improved simulations of these phenomena. Non-adiabatic effects are key to understanding photoinduced dynamics in biological chromophores, photovoltaic materials, and catalytic systems, where they impact energy dissipation and functional efficiency. However, as molecular size increases, computational costs become prohibitive, requiring approximations to make such studies feasible.

Surface hopping dynamics emerged as a practical solution, often combining time-dependent methods for systems in which CASSCF and non-adiabatic coupling calculations are expensive. Recently, we introduced the NORA approach, which performs surface hopping dynamics within a molecular mechanics framework. NORA relies on excited- and ground-state parametrization based on quantum mechanical potential energy surfaces, thereby creating accurate force fields that reproduce photostationary states and other photochemical properties. Initially presented as a proof of concept, we aim to expand this methodology through alternative parametrizations and, importantly, to incorporate non-adiabatic coupling terms to achieve truly non-adiabatic dynamics at a classical level.

This contribution reviews the progression from high-cost CASSCF non-adiabatic molecular dynamics to more affordable methods under development. Examples include CASSCF NAMD on mycosporine-like amino acids and ongoing studies using TD-DFT-based surface hopping, where energetic gaps are used to approximate hopping events in regions that TD-DFT cannot fully describe. Finally, I present NORA as an entry point for excited-state dynamics in biological systems using classical molecular mechanics