Non-Born-Oppenheimer chemistry: Coupled potential energy surfaces and semiclassical trajectories

Non-Born-Oppenheimer processes (also called electronically nonadiabatic processes) are those in which nuclear motion induces a nonradiative change in the electronic state of the system. This thesis presents the results of theoretical and computational studies in three areas related to non-Born-Oppenheimer chemistry: (i) coupled potential energy matrices (PEMs), (ii) accurate quantum mechanical scattering calculations, and (iii) semiclassical trajectory methods for dynamics. Chapter 1 is an introduction. Chapter 2 presents PEMs, accurate quantum mechanical scattering calculations, and semiclassical trajectory calculations for a family of triatomic model systems with two weakly-coupled electronic states. The problem of energetically forbidden surface hops in the semiclassical trajectory surface hopping approach is discussed. Chapter 3 presents PEMs for the LiFH and NaFH systems and an application of the semiclassical trajectory approach to the photodissociation of LiFH and NaFH excited-state complexes. Trends in the lifetimes and reaction probabilities are related to features of the PEMs. Chapter 4 presents an improved semiclassical trajectory algorithm designed to lessen the errors that result from classically forbidden surface hops. The new method is shown to be the most accurate of several methods tested. Chapter 5 presents an analytic global PEM for the two lowest-energy electronic states of LiFH. The fit is based on high-level electronic structure calculations over a dense grid of nuclear geometries. Chapter 6 presents an improvement to the trajectory surface hopping method discussed in Chapter 4. Chapter 7 presents subthreshold and near-threshold quantum mechanical scattering calculations for the ground-state reaction Li + BF → LiF + H. Trends in the lifetimes of the series of resonances observed in the reaction profile are explained by associating the resonance features with quasibound states of the Li···FH van der Waals well.