Preferred basis in coupled electron–nuclear dynamics

Beyond the adiabatic regime, our understanding of quantum dynamics in coupled systems remains limited, and the choice of representation continues to obscure physical interpretation and simulation accuracy. Here, we propose a natural and efficient basis for electron–nuclear dynamics by drawing on the concepts of pointer and preferred states from decoherence theory, adapted to systems where electrons and nuclei interact strongly. Within this framework, we show that (1) the independent dynamics exploited by mixed quantum–classical (MQC) methods are best understood as a manifestation of entanglement viewed in a preferred basis, rather than a consequence of decoherence, and (2) the adiabatic Born–Oppenheimer states satisfy the conditions of an approximate preferred basis. This perspective reconciles widely used approximations with a more fundamental structure of the theory. In effect, we revisit MQC methods through the lens of preferred states, clarifying when they succeed and how they can be improved.