Advances in Coupled Cluster Theory

John F. 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 to develop and implement quantum-mechanical methods for doing calculations relevant to the fields of molecular spectroscopy and chemical kinetics. The Stanton group has a longstanding presence in an area that can be characterized as “Theoretical Chemical Physics”. In the research supported by this grant, they will continue to develop and refine methods relevant to the areas of molecular spectroscopy (how molecules and electromagnetic radiation interact) and chemical kinetics (how quickly chemical reactions occur). In addition to this, the development of the base quantum-chemical methods will be extended. In particular, the supported research will produce cutting-edge software program packages for doing very accurate calculations for “open-shell’’ molecules (those that contain one or more unpaired electrons). It is expected that the software will be the most efficient available to the community for studying this interesting class of molecules that are relevant to areas such as the environment, energy science, and astronomy. This work will extend a highly efficient coupled-cluster (CC) package (NCC, developed previously by the PI and D.A. Matthews of Texas Methodist University) to open-shell reference functions, for which methods for ground and excited states (CC and EOM-CC) will be implemented for general open-shell references up through the CCSDTQ level of theory. In addition, analytic gradients for all of these methods will become available within the funding period. Beyond this, a method that accounts for quantum tunneling effects in chemical kinetics – semiclassical transition theory – will be studied to assess how far below the barrier this method is able to provide accurate results. Remaining research on electronic spectroscopy will range from a diagrammatic fundamental analysis of the spectroscopic intensity of complicated vibronic interactions and study of the uniquely complex A-state of the nitrate radical. 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.