A unified framework for quantum chemistry beyond the Born-Oppenheimer approximation

在量子化学中，量子力学的近似公式应用于原子、分子、表面和反应的行为。该领域由少数关键近似控制。波恩-奥本海默近似，其中分离电子和原子核的运动，具有特别重要的意义，因为它不仅为建模和仿真工具提供了强大的框架，而且还为我们理解分子结构提供了核心理论基础。然而，波恩-奥本海默近似在几个重要的化学、物理和技术相关的背景下被打破：关键的例子包括几乎所有的光激活过程、电化学反应、电荷和能量的传递，以及氢原子迁移的化学反应。由于这个原因，在非绝热动力学方面有大量的研究活动，旨在超越玻恩-奥本海默近似。这项工作的大部分都解决了由于引入势能面而引起的复杂性，势能面的引入将问题从哈密顿量是单粒子和双粒子项的总和改变为所有核自由度耦合在一起的哈密顿量。在这里，我们建议在不引入势能面的情况下超越玻恩-奥本海默近似。本提案的中心思想是发展耦合电子振动自由度的量子化学，最终发展了时间相关和线性响应耦合簇理论，这些理论捕获了非绝热过程中的关键效应。在描述非绝热动力学的同时，在理论的耦合团簇水平上描述电子结构将预示着这类过程建模的新时代，我们将在光化学和振动谱预测方面进行具有挑战性的初步应用，以说明该方法的潜力。

In quantum chemistry the approximate formulations of quantum mechanics are applied to the behaviour of atoms, molecules, surfaces and reactions.The field is dominated by a small number of key approximations. The Born-Oppenheimer approximation, in which a separation is made between the motion of electrons and nuclei, has particular importance because it provides not only a powerful framework for modelling and simulation tools, but also the central theoretical foundation on which our understanding of molecular structure is based. However,the Born-Oppenheimer approximation breaks down in several important chemically, physically and technologically relevant contexts: key examples include practically all photo-activated processes, electrochemical reactions, transport of charge and energy, and chemical reactions where hydrogen atoms migrate. For this reason there is a huge research activity in nonadiabatic dynamics, aimed at moving beyond the Born-Oppenheimer approximation. Much of this work addresses the complexities arising from the introduction of the potential energy surface, whose introduction changes the problem from one where the Hamiltonian is a sum of one- and two-particle terms, to a Hamiltonian in which all nuclear degrees of freedom are coupled together. Here we propose to move beyond the Born-Oppenheimer approximation without introducing potential energy surfaces. The central idea of this proposal is to develop the quantum chemistry of coupled electronic-vibrational degrees of freedom, culminating in the development of time-dependent and linear-response coupled-cluster theories that capture the key effects in nonadiabatic processes. Representing the nonadiabatic dynamics whilst simultaneously describing the electronic structure at a coupled-cluster level of theory would herald a new era in the modelling of such processes, and we will perform challenging preliminary applications in photochemistry and prediction of vibronic spectra to illustrate the potential of the method.