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Development of a Hierarchy of Theoretical Methods towards In Silico Photochemistry

计算机光化学理论方法体系的发展

基本信息

批准号：
2202831
负责人：
金额：
--
依托单位：
Durham University
依托单位国家：
英国
项目类别：
Studentship
财政年份：
2018
资助国家：
英国
起止时间：
2018 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=studentship-2202831
关键词：
Development Hierarchy Theoretical Methods towards

项目摘要

Computational investigation of the processes occurring in molecules upon absorption of light has been of great interest throughout the years. Several methods have been developed with the goal to provide insight into the time evolution of molecules upon excitation. Usually for these methods increased accuracy comes at the cost of increased computational effort, which renders exact dynamics unfeasible. This project will be centered around three main challenges: testing an efficient method that still provides the proper description of decoherence effects, expanding the framework of nonadiabatic dynamics to also account for the decay through spontaneous emission of photons, and finally furthering the theoretical concept of introducing a time-dependent potential energy surface (PES) as an alternative to the usually used concept of the evolution of a time-dependent wavefunction on time-independent PES.One widely used approach for the simulation of nonadiabatic dynamics consists of mixed quantum/classical methods. At the lower end of these methods surface hopping is found, which is computationally feasible even for larger, high-dimensional systems, but comes with a known problem: The phenomenon of decoherence does not arise naturally in this framework leading to "overcoherence". This obstacle has only been circumvented by introducing different ad hoc corrections.In contrast, ab-initio multiple spawning (AIMS) presents a method that correctly predicts decoherence effects during nonadiabatic processes. In AIMS the classical trajectories are substituted by coupled gaussian trajectory basis functions (TBF), which enables an accurate description of quantum effects. However, the increase in accuracy comes at the cost of computational efficiency. An approach has recently been proposed to alleviate this computational bottleneck, where uncoupled groups of coupled TBFs are detected and reduced to only one group, based on a stochastic selection process, the stochastic selection- (SS-) AIMS.In the first part of the project, the surface hopping dynamics of several molecules will be investigated with and without using decoherence corrections. In comparison, the dynamics of the same molecules will be studied employing AIMS as well as SS-AIMS, to get insight into the performance of SS-AIMS, as compared to surface hopping and AIMS.Despite the high accuracy of AIMS simulations, there are still processes that cannot be described by the method, e.g., there does not yet exist a framework of AIMS that would be able to take spontaneous emission of an excited atom into account. The description of spontaneous transitions from an excited state to the ground state while emitting a photon, necessitates the presence of a quantized electromagnetic field. However, the framework of the Schrödinger equation only accounts for the quantisation of electronic energy levels. Hence, it has to be extended to a quantum field theory where the electromagnetic field is quantised, and the Hamiltonian includes the atoms as well as the field and the coupling between them. The next part of the project consists of the development and implementation of an extension of AIMS to quantum electrodynamics with the goal to enable the consideration of spontaneous emission.The above-stated methods for describing nonadiabatic dynamics are all based on the so-called Born-Huang ansatz, where the molecular wavefunction is expressed into a basis of electronic eigenstates. An alternative to this representation of the wavefunction is provided by the exact factorisation, where the molecular wavefunction is expressed as a single product of a nuclear wavefunction and an electronic factor. The resulting concept of a time-dependent PES can be used as a starting point for the development of simpler ways to simulate nonadiabatic processes. The final part of the project would be dedicated to further theoretical approaches based on the exact factorisation of the wavefunction.

多年来，对分子吸收光的过程的计算研究一直是人们非常感兴趣的问题。为了洞察分子在激发时的时间演化，已经开发了几种方法。通常，对于这些方法来说，提高精度是以增加计算工作量为代价的，这使得精确的动力学变得不可行。这个项目将围绕三个主要挑战展开：测试一种仍然能正确描述退相干效应的有效方法，扩展非绝热动力学的框架以解释光子自发辐射的衰变，以及最终深化引入含时势能面(PES)的理论概念，作为通常使用的含时波函数在含时势能面(PES)上演化的替代概念。一种广泛用于模拟非绝热动力学的方法包括量子/经典混合方法。在这些方法的最低端，发现了表面跳跃，这在计算上甚至对更大的高维系统也是可行的，但伴随着一个已知的问题：在这个框架中，退相干现象并不是自然出现的，从而导致“过度相干”。这一障碍只能通过引入不同的特别修正来绕过。相反，从头计算多重产卵(AIMS)提供了一种正确预测非绝热过程中的退相干效应的方法。在AIMS中，用耦合的高斯轨迹基函数(TBF)代替经典的轨迹，从而能够准确地描述量子效应。然而，精度的提高是以计算效率为代价的。最近提出了一种方法来缓解这种计算瓶颈，在这种方法中，基于随机选择过程-(SS-)AIM，检测到耦合的TBF的未耦合的群并将其减少到只有一群。在项目的第一部分，将研究几个分子的表面跳跃动力学在有和没有使用退相干校正的情况下。相比之下，将使用AIMS和SS-AIMS来研究相同分子的动力学，以深入了解SS-AIMS的性能，与表面跳跃和AIM相比。尽管AIMS模拟的精度很高，但仍然有一些过程不能用该方法来描述，例如，还不存在能够考虑激发原子自发辐射的AIMS框架。在描述发射光子时从激发态到基态的自发跃迁时，必须存在一个量子化的电磁场。然而，薛定谔方程的框架只解释了电子能级的量子化。因此，它必须扩展到量子场论，其中电磁场是量子化的，哈密顿量包括原子和场以及它们之间的耦合。该项目的下一部分包括开发和实现AIMS到量子电动力学的扩展，目标是能够考虑自发辐射。上述描述非绝热动力学的方法都基于所谓的Born-Huang ansatz，其中分子波函数被表示为电子本征态的基。波函数的这种表示的另一种选择是由精确因式分解提供的，其中分子波函数被表示为核波函数和电子因子的单个乘积。由此产生的依赖于时间的PES的概念可以作为开发更简单的方法来模拟非绝热过程的起点。该项目的最后部分将致力于基于波函数的精确因式分解的进一步的理论方法。