X-ray absorption and infrared spectra of water and ice: A first-principles electronic structure study

Water is of essential importance for chemistry and biology, yet the physics concerning many of its distinctive properties is not well known. In this thesis we present a theoretical study of the x-ray absorption (XA) and infrared (IR) spectra of water in liquid and solid phase. Our theoretical tools are the density functional theory (DFT), Car-Parrinello (CP) molecular dynamics (MD), and the so-called GW method. Since a systematic review of these ab initio methods is not the task of this thesis, we only briefly recall the main concepts of these methods as needed in the course of our exposition. The focus is, instead, an investigation of what is the important physics necessary for a better description of these excitation processes, in particular, core electron excitations (in XA) that reveal the local electronic structure, and vibrational excitations (in IR) associated to the molecular dynamics. The most interesting question we are trying to answer is: as we include better approximations and more complete physical descriptions of these processes, how do the aforementioned spectra reflect the underlying hydrogen-bonding network of water?;The first part of this thesis consists of the first four chapters, which focus on the study of core level excitation of water and ice. The x-ray absorption spectra of water and ice are calculated with a many-body approach for electron-hole excitations. The experimental features, even the small effects of a temperature change in the liquid, are reproduced with quantitative detail using molecular configurations generated by ab initio molecular dynamics. We find that the spectral shape is controlled by two major modifications of the short range order that mark the transition from ice to water. One is associated to dynamic breaking of the hydrogen bonds which leads to a strong enhancement of the pre-edge intensity in the liquid. The other is due to densification, which follows the partial collapse of the hydrogen bond network and is responsible for the substantial change of the main spectral edge in the conversion from ice to water. The effect of densification may not involve hydrogen bond breaking as shown by experiment in high-density amorphous ice.;Chapter 1 serves as a short summary of the problems at hand, as well as an outline of our theoretical and numerical approaches. In Chapter 2, some necessary background of the subject is provided, including a review of the water XAS controversy in the literature. Chapter 3 describes how we compute XAS theoretically from ground state DFT. In Chapter 4, we go beyond ground state theories and investigate the quasi-particle behavior of the excited states. This is achieved using the many-body Green's function approach known as the GW approximation (GWA). We also present a detailed analysis of the origin of calculated spectral features.;The second part of this thesis, Chapter 5, is devoted to a discussion of the infrared spectra of ice and water, in particular, the role of dynamic dipolar correlations. In this chapter, we present a method to decompose the dipolar correlations into intra- and intermolecular contributions. We find that intermolecular contributions play a role as important as the intramolecular counterpart over the entire frequency range. The decomposition technique proposed here is generally applicable to interpret other spectroscopic data and other systems, such as water at interfaces.;The conclusions of our studies are summarized in Chapter 6.