New Algorithms and their Use in the Study of Nonadiabatic Processes Influenced by Conical Intersections

约翰霍普金斯大学的David Yarkony得到了化学系化学理论、模型和计算方法项目的支持，该奖项旨在开发理论和计算方法来研究分子通过吸收光能而激发的化学反应。科学家可以将化学反应描述为大理石在表面上滚动：反应物点A；产物点B；下山容易(提取化学能)；上山难(需要增加能量)。对于许多光引发的过程，如光合作用、视觉和太阳能转换，这一概念必须得到推广。在这种情况下，由于吸收光线，大理石从更高的表面开始。科学家们想知道大理石是如何回到下表面的，它最终在那个表面的什么地方，也就是说，是什么分子组成的，它们有多少能量。如果光是由激光提供的，那么手头的任务可能是激光控制化学反应。如果光线来自太阳，能量转换、光合作用或细胞损伤可能是问题所在。从上表面到下表面的通道与沿排水沟螺旋而下的通道相似--它涉及一种双圆锥形结构(两个圆锥体在它们的顶点连接在一起)的漏斗，称为“圆锥形交叉点”。由此产生的多曲面过程的数学描述需要曲面和连接圆锥体的详细知识。对表面和圆锥体的描述越准确，计算机模拟就越有可能符合现实。这个项目的主要目标是使用现有最精确的工具来表示表面和连接圆锥体，并使计算机能够可靠地模拟能量过程。非绝热动力学中的一个主要问题是所使用的电子结构数据(能量和州际耦合)的质量。当采用直接或动态动力学技术时，电子结构数据的质量从属于快速获得该数据的需要。另一种选择是使用拟合的耦合势能面。这种方法可以比直接动力学方法更准确地使用电子结构描述，并可以平滑由于非绝热过程中固有的轨道变化而可能出现的电子能量的不规则性。挑战在于确定合适的人选。通过将最小二乘拟合与精确的数据表示相结合，并将导数耦合数据直接纳入到拟合过程中，发展了一种算法，该算法提供了电子结构数据的可量化的准非绝热拟合表示，包括对锥形相交接缝的可靠描述。该方法适用于4-5原子系统。这项研究的目标是将该算法扩展到处理更大的分子，例如苯酚的光解离。提出的Fit曲面算法将使高质量的电子结构数据可用于研究核碱基和许多大小相当的系统中的非绝热动力学。确定这种方法可以扩展到什么程度，也就是一个系统可以被处理多大，是这项研究的主要目标之一。

David Yarkony of Johns Hopkins University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry Division to develop theoretical and computational approaches to study chemical reactions that are initiated when a molecule is excited by absorbing energy from light. Scientists can describe a chemical reaction as a marble rolling on a surface: reactants point A; products point B; moving downhill is easy (extract chemical energy); moving uphill is hard (need to add energy). For many light-initiated processes such as photosynthesis, vision, and solar energy conversion, this notion must be generalized. In this case, as the result of absorbing light, the marble starts on a higher surface. Scientists want to know how the marble gets back to the lower surface and where it ends up on that surface, that is, what molecules are made and how much energy they have. If the light is provided by a laser, the task at hand may be laser control of a chemical reaction. If the light comes from the sun, energy conversion, photosynthesis or cell damage may be the issue. The passage from the upper surface to the lower surface not unlike spiraling down a drain - it involves funneling by a double cone-like structure (two cones joined at their vertices) called a "conical intersection". The mathematical description of the resulting multi-surface process requires detailed knowledge of the surfaces and the connecting cones. The more accurate the description of the surfaces and cones the more likely the computer simulation will correspond to reality. The principal goal of this project is the representation of the surfaces and connecting cones using the most accurate tools available and enabling computers to reliably simulate energetic processes. A principal issue in nonadiabatic dynamics is the quality of the electronic structure data (energies and interstate couplings) used. When direct or on-the-fly dynamics techniques are employed, the quality of electronic structure data is subordinated to the need to obtain that data quickly. The alternative is to use fitted coupled potential energy surfaces. This method can use electronic structure descriptions more accurately than those used in direct dynamics and can smooth the irregularities that may occur in the electronic energies due to the orbital changes inherent in nonadiabatic processes. The challenge is to determine the fit. By combining least squares fitting with exact data representation and incorporating derivative coupling data directly into the fitting process, an algorithm has been developed that provides a quantifiably quasi-diabatic fitted representation of electronic structure data including a reliable description of seams of conical intersections. That approach is suitable for 4-5 atom systems. The goal of this research is to extend this algorithm to treat significantly larger molecules, such as the photodissociation of phenol. The proposed fit surfaces algorithm will make high quality electronic structure data available for the study of nonadiabatic dynamics in nucleobases and a multitude of comparably sized systems. Determining how far this methodology can be extended, that is how large a system can be treated, is one of the principal goals of this research.