Advances in Coupled-Cluster Theory with Molecular Applications

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John Stanton of the University of Florida is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry. Stanton and his research group work in the field of theoretical chemical physics, which aims to apply concepts from physics to the study of atoms and molecules. They work in two research areas: quantum chemistry and molecular dynamics. In quantum chemistry, the Stanton group develops and applies computational tools from the class of methods for treating electrons that is known as coupled-cluster theory to enable highly accurate calculations of the properties of molecules that are of interest to experimentalists. In molecular dynamics, they develop approaches to semiclassical transition state theory, enabling the prediction and understanding of the rates at which chemical reactions take place. Potential applications of the quantum chemistry research include very detailed calculations to improve the fidelity of chemical models to study chemical processes in the atmosphere and industry. By combining the work in quantum chemistry with the reaction rate theory developments, they can perform accurate studies of chemical reactions involving radicals and biradicals. This latter area of research has significant import in many areas, since the oxygen molecule so vital to life and which constitutes 20% of the atmosphere is a biradical. The quantum chemistry methods that are developed in this research are all implemented in the CFOUR quantum chemistry package, which is freely available to the research community. Stanton is working to develop an honors program at his university for highly-motivated and highly-accomplished students enrolled in the study of the natural sciences.The proposed research involves explorations in the coupled-cluster treatment of electronic correlation, specifically involving high-accuracy methods including effects of quadruple excitations and the multireference effects that are characteristic of the ubiquitous (and important) class of problems that comprise "biradical" systems. In addition, work will be done to explore the accuracy and limits of semiclassical transition state theory, involving: benchmarking against exact quantum results for multidimensional model systems, designing extensions of the method, and exploring the formal connections between the pseudo-states treated by this method and Siegert eigenstates. In addition, work will be done on developing methods for the treatment of vibronic coupling in molecules that include angular momentum effects, culminating in the design and implementation of a spin-rovibronic treatment.