Zhang Yu, Li Linqiu, Tretiak Sergei, Nelson Tammie
Physics and Chemistry of Materials, Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States.
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
J Chem Theory Comput. 2020 Apr 14;16(4):2053-2064. doi: 10.1021/acs.jctc.9b00928. Epub 2020 Mar 23.
Nonadiabatic Molecular Dynamics (NAMD) of excited states has been widely used in the simulation of photoinduced phenomena. However, the inability to treat bond breaking and forming processes with single-reference electronic structure methods limits their application in photochemistry for extended molecular systems. In this work, the extension of excited-state NAMD for open-shell systems is developed and implemented in the NEXMD software. We present the spin-unrestricted CIS and TD-SCF formalism for the ground and excited states, analytical derivatives, and nonadiabatic derivative couplings for the respective potential energy surfaces. This methodology is employed to study the photochemical reaction of three model molecules. The results demonstrate the advantage of the open-shell approach in modeling photochemical reactions, especially involving bond breaking processes. We find that the open-shell method lowers the reaction barrier at the bond-breaking limits resulting in larger calculated photochemical quantum yields compared to the respective closed-shell results. We also address problems related to spin contamination in the open-shell method, especially when molecular geometries are far from equilibrium.
激发态的非绝热分子动力学（NAMD）已广泛应用于光诱导现象的模拟。然而，单参考电子结构方法无法处理键的断裂和形成过程，限制了它们在扩展分子系统光化学中的应用。在这项工作中，开壳层系统激发态NAMD的扩展在NEXMD软件中得以开发和实现。我们给出了基态和激发态的自旋非限制CIS和TD-SCF形式、解析导数以及各自势能面的非绝热导数耦合。该方法用于研究三个模型分子的光化学反应。结果表明开壳层方法在模拟光化学反应方面的优势，特别是涉及键断裂过程。我们发现，与相应的闭壳层结果相比，开壳层方法降低了键断裂极限处的反应势垒，从而导致计算出的光化学量子产率更高。我们还解决了开壳层方法中与自旋污染相关的问题，特别是当分子几何结构远离平衡时。
