CDS&E: Advances in Coupled-Cluster Theory with Molecular Applications

University Of Texas At Austin, Austin TX

Investigators

Abstract

John Stanton of the University of Texas at Austin is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry Division and the Computational and Data-Enabled Science and Engineering program (CDS&E) to develop theoretical methods and software to describe the properties of molecules in which the electrons have been excited. The major thrust of the work is to develop and use quantum mechanical methods to understand experiments in the field of molecular spectroscopy and to predict the results. Spectroscopy probes how molecules interact with electromagnetic radiation. By investigating how a particular molecule behaves at various wavelengths (energies) of irradiation, one can determine the structure of the molecule as well as infer most of the detailed information that governs the chemical reactivity and other behavior of the molecule. A major goal of the project is to develop quantum-mechanical methods that are useful for analyzing the spectra of molecules in the visible and ultraviolet energy ranges, which is the most relevant region for the electrons that drive chemistry. The computational methods and tools developed by the Stanton group are embodied in a free and publicly available computer program, CFOUR, which computes very accurate quantities that can be compared with experiment. This computer code takes advantage of state-of-the-art computer architecture. Both graduate and undergraduate students are involved in these projects. The work has relevance to many fields including interstellar and atmospheric chemistry. In terms of traditional quantum chemistry, Stanton and his research group focus on methods development at very high levels of coupled-cluster theory, specifically those that involve quadruple excitation effects. This includes methods for energy and gradient calculations, as well as extension to radicals, ions and excited states that can be accessed by the equation-of-motion approach. All methods are incorporated into the CFOUR program package, leading to new program modules that will exploit advantages offered by massively parallel computers. Additional research focuses on non-adiabatic effects in spectroscopy, both in the development of interpretive tools as well as development of programs. While computer program development is a major focus of the planned research, the aforementioned investigation of interpretive tools is not computational in nature. Similar in his respect are two other project activities: the establishment and benchmarking of a tool for analyzing the adequacy of particular levels of coupled-cluster theory for treating specific problems; and analysis of some formal properties of Miller's semiclassical transition state theory.