Non-Born-Oppenheimer Effects in the Framework of Multicomponent Time-Dependent Density Functional Theory

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Sharon Hammes-Schiffer of Yale University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to develop computational approaches to study the interaction between electrons and protons in chemical systems. The coupling between electrons and protons plays a vital role in many important biological processes such as photosynthesis and respiration. They also play a role in technology, for example, in energy production in solar cells. Developing computational methods to accurately describe this coupling is challenging. Electrons and protons are so light that they must be treated quantum mechanically. Hammes-Schiffer and her coworkers develop quantum mechanics methods that are computationally efficient. These methods are applied to specific processes of biological and chemical relevance. The Hammes-Schiffer research group also maintains and enhances a website containing software and educational tools related to this topic. The computer programs, tools, demonstrations, and tutorials available on this web site will enable scientists in a broad range of fields to learn about this topic. In addition, this project facilitates technological and biomedical advances through a better understanding of the coupling between electrons and protons. An important example is the design of more effective solar cells and other alternative, renewable energy sources. Another example is the design of more effective drugs through modification of enzymes that rely on the coupling between electrons and protons. The objective of this project is to develop new theoretical and computational approaches that will provide insight into the underlying fundamental principles of photoinduced proton transfer and proton-coupled electron transfer (PCET) reactions, which play a vital role in a broad range of biological and chemical processes. The specific issues to be examined include the roles of nuclear quantum effects, such as proton delocalization and zero-point energy, as well as non-Born-Oppenheimer effects, which are often significant in these types of reactions. The method development is conducted within the framework of the nuclear-electronic orbital density functional theory (NEO-DFT) approach, which treats key nuclei, such as the transferring proton(s), quantum mechanically on the same level as the electrons within the framework of DFT. The focus is to develop the multicomponent time-dependent DFT (NEO-TDDFT) approach for calculating excited electronic, proton vibrational, and electron-proton vibronic states. This approach is used to compute excitation energies and transition densities for photoinduced proton transfer and PCET systems. The NEO-DFT and NEO-TDDFT approaches is being incorporated into a publicly available electronic structure package. In addition, tutorials are being created to explain how to perform NEO calculations and to highlight the unique capabilities of this approach. Furthermore, a web site on PCET is being enhanced to convey useful information to the community and to provide useful tools, scripts, and programs relevant to studying PCET.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.