Theory and Application of Some New Approaches to Electronically Non-Adiabatic Dynamic

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William H. Miller and Stephen J. Cotton of the University of California, Berkeley are supported by an award from the Chemical Theory, Models and Computational Chemistry Program in the Division of chemistry to develop theoretical methods for simulating a variety of molecular processes that are fundamental to many modern technologies. These processes involve coupled electronic states. Such simulations allow researchers to understand these processes more deeply and thus be able to modify them in useful ways. Examples are biomolecular processes, photovoltaic behavior, and photo-catalysis. The approach that Miller and Cotton is developing is characterized by its simplicity and its efficiency. The goal is to have a reliable and easy to use approach for simulating larger and more complex molecular systems than is what is currently possible. The specific focus of this research is electronically non-adiabatic processes, i.e., those that involve transitions between different electronic states. The challenge is to treat the electronic degrees of freedom and the nuclear degrees of freedom, to which they are coupled, in a consistent framework. The approach uses the Meyer-Miller (MM) model for the electronic degrees of freedom, which shows how they can be "mapped" onto harmonic oscillators, one for each electronic state. This then allows the use of classical molecular dynamics for electronics as well as nuclear degrees of freedom. The MM model would be exact if all the degrees of freedom were treated quantum mechanically. Treating them all classically is an approximation but can often be very accurate by using semiclassical ideas to "re-quantize" the electronic degrees of freedom This has resulted in the development of a striking simple symmetrical quasi-classical (SQC) windowing procedure that provides a semiclassical-like "quantization" of the electronic oscillator degrees-of- freedom (DOF) in classical Meyer-Miller (MM) vibronic dynamics. Significant benchmarking of this SQC/MM approach has been seen to give semiclassical-like (and therefore quantum-like) results, but to do so orders of magnitude more efficiently than a true semiclassical calculation and, of course, without the inherent exponentially scaling cost of a full quantum dynamics simulationThis 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.