QUANTUM AND SEMICLASSICAL METHODS FOR RATE CONSTANT CALCULATIONS

Chemical reactions are intrinsically dynamical processes. Reaction rate constants, and thus the understanding of chemical kinetics, can be in principle obtained at a very detailed level if one is able to compute the real time quantum dynamics for the reactive system. Unfortunately, the numerical implementation of real time quantum dynamics is very hard to perform, especially for high dimensional systems, because the computational effort scales exponentially with the number of degrees of freedom. In this Ph.D. thesis, two open problems in reaction rate theory have been addressed. The first one is to extend to high dimensional systems the inclusion of quantum effects in rate constant computations. The second issue deal with the inclusion of real time dynamics into very accurate rate constants calculations. The thesis is organized as follows. After a general Introduction, the second chapter is an overview of the state of the art in reaction rate theory. Then, in the third chapter, the derivation of Miller's Semiclassical Transition State Theory (SCTST) is recalled. SCTST is the method employed to obtain accurate and quantum-corrected rate constants for high dimensional reactions. In chapter 4, a novel parallel implementation of this theory (that has also been released as an open source code into J. R. Barker's MultiWell suite of codes) is described together with its application to high dimensional systems. In the following chapters, a new quantum rate approach able to include real time dynamics effects is presented. Derivation and applications of the latter are thoroughly described in chapter 6. The thesis ends with some perspectives about possible future developments.