Preferred Basis in Coupled Electron-Nuclear Dynamics

TL;DR

This paper introduces a natural basis for electron-nuclear quantum dynamics inspired by decoherence theory, clarifying the role of entanglement and adiabatic states in mixed quantum classical methods to improve their reliability.

Contribution

It proposes a preferred basis framework for coupled electron-nuclear dynamics, linking MQC methods to decoherence concepts and showing how adiabatic states serve as an approximate preferred basis.

Findings

Independent dynamics in MQC methods relate to entanglement in a preferred basis.

Adiabatic Born-Oppenheimer states approximate a preferred basis.

The framework offers a systematic way to enhance MQC strategies.

Abstract

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 is 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 and provides a systematic route to more reliable MQC strategies. In effect, we revisit MQC methods through the lens of preferred states, clarifying when they succeed and how they can be improved.