Fragment Embedding for Photochemical Electronic Structure Simulations

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Professor Troy Van Voorhis of Massachusetts Institute of Technology is supported by an award from the Chemical Theory, Models and Computational Methods Program in the Division of Chemistry. At a microscopic level, chemical reactions determine how long light emitting diodes (LEDs), solar cells, and batteries will last. Recently it has become possible - and even routine - to make predictions about useful lifetimes using computer simulations. These simulations complement experiments by providing insight into properties that might difficult to measure directly. This project considers the use of embedding computer methods that may make it possible to simulate much larger systems with much higher accuracy than ever before. The fundamental idea of embedding involves breaking a large system up into many small, overlapping fragments that can then be stitched back together to describe the whole. The Van Voorhis group uses these embedding tools to study the photochemistry of molecules relevant to emerging organic light emitting device (OLED) and solar cell technologies. Professor Van Voorhis is the departmental coordinator of MIT ACCESS - a joint program between Chemistry, Chemical Engineering and Materials Science that invites a select group of underrepresented minority undergraduate students to visit MIT for a weekend during the academic year. The primary aim is to encourage the students to consider graduate school in chemistry. Professor Van Voorhis also empowers and equips graduate students within his group to introduce K-12 students to the beauty and power of science. Graduate and undergraduate students involved in this award are encouraged to participate in the MIT chemistry "Magic Show" that performs at local schools; the Research Communication Laboratory that trains graduate students to communicate scientific concepts to K-12 students; MIT Women in Chemistry's "Scientist for a Day" middle school camp; and the MIT museum's "Science on Saturdays" program that targets children and families. The Van Voorhis group is developing transformative electronic structure methods in which fragments are the fundamental building blocks. The goal is to create new tools that are both faster and more accurate than state-of-the-art approximations, facilitating chemical simulations that were previously inaccessible. As a starting point, they develop a family of fragment embedding theories based on the Bootstrap Embedding method. The key realization is that wave function embedding allows one to combine overlapping orbital fragments in a robust way. The resulting theory circumvents the scaling challenges inherent in most wave function theories by only requiring calculations on very small fragments, which are ultimately combined to obtain a picture of the entire system. Improvements are accomplished by dressing these embedding methods with dynamic correlation using the many pair expansion (MPE). Finally, these techniques are extended to excited states using a novel strategy that identifies such states as local minima of the energy variance. As a signature application of these tools, the Van Voorhis Group studies the energetics and dynamics of triplet excitons in molecules relevant to emerging OLED and solar cell technologies.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.