Garnet Chan of Cornell University is supported by the Theoretical and Computational Chemistry program, with partial support from the Materials Theory program, for research to enable accurate calculations of excited states of large molecules. The PI and his group are constructing a new approach to multireference quantum chemical problems based on Renormalization Group ideas. Specifically, they are investigating excited state and response theories for joint Density Matrix Renormalization Group and Canonical Transformation calculations. Reduced-scaling algorithms for these theories are enabling accurate descriptions of complicated excited states that are not described correctly by traditional methods. Applications include spectra and properties of pi-conjugated organic molecules of materials and biological interest, such as the chromophores involved in vision and light-harvesting. The PI is implementing an educational program to integrate "investigative" scientific computing into the undergraduate and graduate curriculum. He is also implementing several projects to bring an understanding of the quantum world and nanomaterials to children. These include a nanotechnology exhibit and quantum simulator at a local science museum. This work is having a broader impact in the development of techniques to study the excited states of molecules and materials, training of undergraduates and graduate students in computational science, and outreach to the K12 and wider community.
How does a plant absorb light and convert it to sugar? How can we design materials that similarly harness the power of the sun to power our technologies? These questions require us to understand the interaction between light and matter, and the influence of light on chemistry. Electronic structure theory seeks to predict the course of chemical reactions through the laws of quantum mechanics, in particular by using quantum mechanics to predict the motion of electrons, that are the chemically active constituents of molecules. However, the standard mathematical approximations break down when light is involved, because molecules are transformed into unusual, so-called excited, states. The funded research aimed at developing new mathematical approximations that, in conjunction with the application of modern computing power, would more accurately describe the quantum mechanical behaviour of molecules and their constituent electrons in the presence of light. Over the last five years we developed a set of theoretical techniques built around the idea of renormalization group theory. This theory was originally developed in the context of Nobel Prize winning work that led to the understanding of large assemblies of molecules. However, as we have shown, modern variations of the idea are also extremely useful in understanding the large assemblies of electrons that are present in molecules. Using these techniques to model electrons in molecular excited states, we found that the excited states of photo-active molecules contain unusual collective motions of electrons. It is this collective motion, similar to a "mexican wave" in a crowd or a stadium, which is behind the failure of the standard mathematical approximations, which assume that electrons behave relatively independently of each other. At the same time, we argue that is these unusual "dark" states that are actually responsible for the remarkable efficiency of nature's light harvesting apparatus. We also found that many classes of molecules used in modern organic electronic materials also contain similar kinds of dark states, which are also central to controlling their interaction with light. These discoveries yield new target criteria to optimize molecular designs to develop efficient solar materials. Outside of the societal impacts related to technologies that benefit from understanding the interaction of matter with light, our educational efforts have targeted audiences ranging from middle school to the college undergraduates. For the former, we have worked with underprivileged Puerto Rican middle schoolers, to introduce them to basic concepts of science through hands-on demonstrations. For the latter, we have developed electronic educational modules that allow an interactive exploration of basic quantum physics.
