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

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

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.