Mixed Quantum–Classical Dynamics for Molecular Excited States: Reliability, Scalability, and Representation

Mixed quantum–classical dynamics (MQCD) enables nonadiabatic simulations of molecular excited states by combining classical nuclear trajectories with quantum electronic‐state population flow. The field is moving beyond benchmark small molecules toward nanoscopic systems, active environments, and long timescales—regimes where MQCD is stretched by three coupled demands: reliability, scalability, and flexibility in the electronic representation set by decoherence. Reliability means getting the controlling physics right: initial ensembles consistent with the experimental preparation, accurate potential energies and couplings driving the system's evolution, and correct nonadiabatic branching and recombination. Scalability demands reducing time‐to‐converged observables by curbing repeated high‐level electronic‐structure calls. Surrogate excited‐state models (including machine‐learning potentials) can replace much of the on‐the‐fly electronic structure, while active learning, multi‐fidelity strategies, and uncertainty quantification guide where new reference data and additional sampling are actually needed. Flexibility of representation is essential in dense excited‐state manifolds, where the decoherence‐selected pointer basis may differ from the adiabatic energy basis; localized representations and open‐quantum‐system tools (Lindblad dynamics, stochastic unraveling) provide practical routes to encode that choice. Together, these directions define an agenda for MQCD that targets larger, longer, and more environmentally complex excited‐state dynamics while retaining a trajectory‐based workflow. This article is categorized under: Quantum Computing > Theory Development Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics