非绝热电子-离子耦合动力学进展

Progress in nonadiabatic coupled electon-ion dynamics

虽然Born-Oppenheimer近似的运用取得了巨大的成功,但是在非绝热跃迁过程中Born-Oppenheimer近似不再成立,所以需要重点描述电子与核之间的耦合运动。在光物理、光化学、强场物理以及离子碰撞中,非绝热过程极为常见。电子激发是非绝热过程的核心,这些过程与基态和激发态均有密切关系。原则上全量子力学方法可以准确处理非绝热电子–离子耦合,但是目前只局限于两三个原子构成的小体系。严格求解量子力学所需的计算量随着体系的增大而呈指数级增加,因此精确的量子力学处理多原子体系的非绝热过程是非常困难的。近几十年来,国内外多个小组发展了许多不同的方法来研究非绝热过程。本文重点介绍了非绝热混合量子–经典方法,即只对原子核部分的量子力学作经典极限,保留电子的量子效应。常用的混合量子–经典方法包括了Ehrenfest方法、面跳跃方法、杂化方法(即结合了Ehrenfest和面跳跃优点的方法)、基于Wigner转换的刘维尔方法。但是当原子核的量子效应显著时,非绝热动力学必须通过其它更严格的方法处理,比如从头计算多次产生方法。本文希望向刚接触非绝热动力学的同行简洁明了地阐述以上各种方法的基本思想、数值实现过程、优缺点和最新研究进展。

The application of Born-Oppenheimer approximation has received a great success, however it fails to treat dynamical process involving nonadiabatic transition. Therefore, the description of ionic motion coupled with electrons must be taken into account. The nonadiabatic process is extremely common in the field of photophysics, photochemistry, strong-field physics and ion col- lisions. Electron excitation is essential in nonadiabatic process, which is closely connected with ground state and excited state. Basically full quantum mechanical methods can deal with nonadi- abatic electron-ion coupling without making any approximations, but it is presently restricted to small systems consisting of two or three atoms. The computational cost increases exponentially with increasing the number of atom when to exactly solve quantum mechanics equations, which is far too difficult to treat nonadiabatic process in atomic system. Over recent decades, several groups have developed many different methods to study the nonadiabatic process. This work mainly introduces the nonadiabatic mixed quantum-classical method, where nuclei are treated with classical limit, and electrons are described by quantum theory. Mixed quantum-classical method includes Ehrenfest method, Surface hopping method, Hybrid method （combined merits of Ehrenfest method with Surface hopping method）, and Liouville method based on Wigner trans- form. However, when the quantum effects of nuclei are remarkable, nonadiabatic dynamics must resort to other more rigorous method, such as ab initio multiple spawning. This article briefly and clearly introduces relevant concepts, numerical implementation, recent progress and its pros and cons.