SusChEM: Stochastic Bethe-Salpeter Approach to Excited States in Large Molecules and Nanocrystals

加州大学伯克利分校的Eran Rabani教授在化学部(CHE)的化学理论、模型和计算方法以及大分子、超分子和纳米化学程序以及材料研究部(DMR)的凝聚态和材料理论程序的支持下，开发了研究大分子和纳米系统中电子激发态的理论和计算方法。开发新型纳米结构材料，如纳米晶体、纳米棒及其复合材料，用于光采集和储能设备，最终目标是制造轻质和高效的设备，引起了人们的极大兴趣。然而，缺乏预测工具来描述这类系统的物理性质一直是设计具有定制性质的纳米材料的主要瓶颈之一。拉巴尼和他的同事试图通过对激发电子态的准确描述来解决这个问题，这是一项相当具有挑战性的理论/计算任务。因此，本研究的目标是开发一种描述激子能级排列和具有计算复杂性的吸收光谱的方法，该方法可扩展到纳米尺度的实验相关系统。这是本方案的主要目标。研究生和博士后研究助理参与了这项研究，并接受了尖端理论和计算方法的培训。为了降低描述光激发态的计算复杂性，Rabani和他的研究小组正在开发基于Bethe-Salpeter方程的实时形式化，Bethe-Salpeter方程是描述扩展系统中激发态的最准确方法之一。Bethe-Salpeter方法在凝聚态物理中相当流行，但在化学中一直没有得到充分的应用。实时形式化允许他们将计算尺度减少到立方，这是一个显著的改进，但对于扩展系统来说是不够的。因此，为了进一步降低计算成本，将努力开发基于Bethe-Salpeter方法的依赖于时间的描述的随机公式，从而导致系统大小的二次缩放。为了检验新理论的准确性，我们将把依赖于时间的随机Bethe-Salpeter方法的预测值与不同尺寸的纳米晶体、纳米棒和种子纳米棒的实验测量量进行比较。如果成功，这些努力将使Bethe-Salpeter方法成为描述扩展分子系统中激发态的便捷工具，就像含时密度泛函理论是研究小分子体系激发态的便捷工具一样。

Professor Eran Rabani of the University of California, Berkeley is supported by the Chemical Theory, Models and Computational Methods and the Macromolecular, Supramolecular and Nanochemistry programs in the Chemistry Division (CHE) and the Condensed Matter and Materials Theory program in the Division of Materials Research (DMR)to develop theoretical and computational approaches to study electronically excited states in large molecular and nanoscale systems. There is significant interest in the development of new nanostructured materials such as nanocrystals, nanorods and their composites for light harvesting and energy storage devices, with the ultimate goal of fabricating lightweight and high efficiency devices. However, the lack of predictive tools to describe the physical properties of such systems has been one of the major bottlenecks in the design of nanomaterials with tailored properties. Rabani and his coworkers seek to address this problem by working toward an accurate description of excited electronic states, so-called "electron-hole excitations" in extended systems a rather challenging theoretical/computational task. It is therefore the goal of this research to develop a means to describe the excitonic level alignment and the absorption spectrum with computational complexity that is scalable to systems of experimental relevance at the nanometer scale. This is the major goal of the present program. Graduate students and postdoctoral research associates are involved in this research and are being trained in cutting edge theoretical and computational methods.To reduce the computational complexity of describing optically excited states, Rabani and his research group are developing a real-time formalism based on the Bethe-Salpeter equation, one the most accurate methods to describe excited states in extended systems. The Bethe-Salpeter approach is rather popular in condensed matter physics but has been underused in chemistry. The real-time formalism allows them to reduce the computational scaling to cubic, which is a significant improvement but not sufficient for extended systems. Thus, to further reduce the computational cost, efforts will be made to develop a stochastic formulation based on the time-dependent description of the Bethe-Salpeter approach, leading to quadratic scaling with system size. To test the accuracy of the new formalism, predictions made by the time-dependent stochastic Bethe-Salpeter approach will be compared with experimental measured quantities on nanocrystals, nanorods, and seeded nanorods of varying dimensions. If successful, these efforts will make the Bethe-Salpeter approach a handy tool to describe excited states in extended molecular systems in much the same way that time-dependent density functional theory is a handy tool for excited states in small molecular systems.