I. Investigations on electronic, vibrational, temperature and isotope effects in hydrogen bonded systems. II. Diabatic extensions to quantum wavepacket ab initio molecular dynamics

Hydrogen bonds are ubiquitous and their properties affect a wide range of areas. Their unique features include binding energies and properties ranging from those found in van der Waals interaction to covalent bonds. These fundamental properties have attracted attentions from numerous experimental and theoretical groups. In the first part of this study, we explore the vibrational properties in hydrogen bonded systems using ab initio molecular dynamics (AIMD). The AIMD approach permits us to investigate hydrogen bonds by including electronic, vibrational, temperature and isotope effects. Several case studies are presented and used to understand experimental results. Specifically, the widely disparate results often seen from infrared multiple photon dissociation and Argon-tagged action spectroscopy experiments are completely understood from the simulations presented here. This is the first time a detailed concordance between these experimental techniques and theory has been achieved. To include quantum nuclear effects, our group has derived a new method called quantum wavepacket ab initio molecular dynamics (QWAIMD). In this thesis, diabatic extensions to QWAIMD are provided. These generalizations directly result from an analysis of the variance in electronic structure with quantum nuclear degrees of freedom. The diabatic electronic states are treated as classical parameters and propagated with a quantum wavepacket and classical nuclei. Substantial reduction in computational costs is achieved compared to previous implementations of QWAIMD.