Exploring Spectral Signatures of Molecular Vibrations

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Professor Anne B. McCoy of the University of Washington is supported by an award from the Chemical Theory, Models and Computational Methods (CTMC) program in the Division of Chemistry to develop new theoretical and computational tools that will be used to explore the manifestations of nuclear quantum effects in hydrogen-bonded systems. Hydrogen-bonding is ubiquitous, and there are often surprisingly large differences between properties of normal water (H2O) and heavy water (D2O). Many of these differences are attributed to the quantum mechanical nature of molecular vibrations. While knowing that these effects exist is easy to articulate, quantifying them is much more difficult. Anne McCoy and her research group develop tools for exploring quantum mechanical effects in molecular vibrations, and through collaborations with experimental groups has worked to enhance our understanding of these effects. The methods that she and her research group are developing can be applied to systems of up to 20 atoms that are small enough for detailed calculations and comparisons to spectroscopic experiments, but large enough to exhibit the types of nuclear quantum effects that affect properties of liquid water or ice. In parallel with this work, Anne McCoy and her group are developing tools that can be used by the community to implement the approaches she has. She is pursuing a variety of activities that focus on development of the next generation of physical chemists including mentoring students and promoting early career scientists through her roles as chair of the International Advisory Committee for the International Symposium on Molecular Spectroscopy and an officer of the American Physical Society’s Division of Chemical Physics.The supported work focuses on the development of vibrational perturbation theory and diffusion Monte Carlo approaches and using these approaches to explore the manifestations of nuclear quantum effects in hydrogen-bonded systems. The work on vibrational perturbation theory focuses on the development of approaches for identifying resonances and exploring the role of third and higher order corrections to the results obtained at second order (VPT2). A key feature of this work is the exploration of the impact of coordinates in which the Hamiltonian is expanded on the insights obtained from vibrational perturbation theory. The work on Diffusion Monte Carlo (DMC) focuses on the development of efficient procedures for evaluating the potential energies used in DMC using neural-network potentials, which are based on the geometries evaluated from small DMC simulations. These potential surfaces are then in larger DMC studies. A central application of these studies is an exploration of how nuclear quantum effects are manifested in water through studies of water clusters. This work focuses on probing relationships between spectra, large amplitude motions and how and why the structure of water and protonated water systems change with deuteration. A theme that underlies these studies is the development of versatile approaches that can be used to explore a broad range of systems, and to develop general understandings that provide broader insights in quantum effects in hydrogen bonding that extend beyond the specific systems that are being studied. The codes developed in this work are part of the PyVibDMC and PyVibPTn codes that Anne McCoy and her research group have been developing.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.