Removing the Barriers to the Calculation of Activation Energies, Activation Volumes, and Mechanistic Insight for Chemical Dynamics

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Ward Thompson of the University of Kansas is supported by an award from the Chemical Theory, Models, and Computational Methods program in the Division of Chemistry to develop methods for predicting the temperature dependence of dynamics in chemical systems. Changing such external properties is a key way to control chemistry in a laboratory or industrial setting. Thus, it is important to understand how these variables affect the timescales of the important motions involved in the chemistry. This knowledge also provides insight into the molecular-level motions that determine the timescale. The methods developed in this project, for example, allow the direct calculation of the temperature dependence of any timescale (such as the time for a chemical reaction to take place) from simulations at a single temperature. At the same time, they give information about the molecular interactions that influence the temperature dependence. Currently, no other method can provide such information. In addition to the behavior with temperature, the effect of other properties such as pressure or system composition may be calculated. While the approaches apply to any chemical system, the Thompson group is focusing on dynamics in liquids. The award also supports an outreach program for elementary school students to increase their interest in and motivation for science. It is designed to let the students see what scientists do and encourage them to view themselves as scientists through activities and interactions with graduate students and faculty. The methods developed fall into three categories: 1) Those enabling the calculation of activation energies for reactions and other dynamical processes from simulations at a single temperature. These approaches provide otherwise unattainable information: the contributions of different interactions to the activation energy. 2) Those evaluating changes in dynamical timescales with physical conditions other than temperature, such as pressure or chemical potential. This makes quantities like the activation volume available from simulations at a single pressure. 3) Approaches for calculating derivatives of rate constants and other dynamical timescales with respect to components of the underlying Hamiltonian, such as masses or potential parameters, to gain insight into mechanisms, force fields, and kinetic isotope effects.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.