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

机译：水和冰的X射线吸收和红外光谱：第一性原理电子结构研究。

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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.
机译：水对于化学和生物学至关重要，但是关于其许多独特性质的物理学尚未广为人知。在本文中，我们对水在液相和固相中的X射线吸收（XA）和红外（IR）光谱进行了理论研究。我们的理论工具是密度泛函理论（DFT），Car-Parrinello（CP）分子动力学（MD）和所谓的GW方法。由于对这些从头算方法的系统性综述不是本论文的任务，因此我们仅简要回顾一下在阐述过程中所需的这些方法的主要概念。相反，重点是研究对于更好地描述这些激发过程，特别是揭示局部电子结构的核心电子激发（在XA中）和与之相关的振动激发（在IR中）必要的重要物理学是什么。分子动力学。我们试图回答的最有趣的问题是：由于我们包括了这些过程的更好的近似和更完整的物理描述，上述光谱如何反映出水的潜在氢键网络？；本论文的第一部分包括前四章着重研究水和冰的核心层激发。水和冰的x射线吸收光谱是通过多体方法计算电子空穴激发的。使用从头算分子动力学产生的分子构型，可以定量定量地再现实验特征，甚至是液体中温度变化的微小影响。我们发现，光谱形状由短程有序的两个主要修改控制，这些修改标志着从冰到水的过渡。一个与氢键的动态断裂有关，这导致液体中的预边缘强度大大增强。另一个是由于致密化，它紧随氢键网络的部分破裂，并导致从冰到水的转换过程中主要光谱边缘的实质性变化。如在高密度非晶冰中的实验所示，致密化的作用可能不涉及氢键的断裂。;第一章是对当前问题的简短总结，也是我们理论和数值方法的概述。在第二章中，提供了该主题的一些必要背景，包括对文献中有关水XAS争议的评论。第3章介绍了我们如何从基态DFT理论上计算XAS。在第四章中，我们超越了基态理论，研究了激发态的准粒子行为。这是使用称为GW近似（GWA）的多体格林函数方法实现的。我们还对计算出的光谱特征的起源进行了详细的分析。本论文的第二部分，第5章专门讨论了冰和水的红外光谱，特别是动态偶极相关性的作用。在本章中，我们提出了一种将偶极相关性分解为分子内和分子间贡献的方法。我们发现，在整个频率范围内，分子间的作用与分子内的对应物一样重要。本文提出的分解技术通常适用于解释其他光谱数据和其他系统，例如界面处的水。;第六章总结了我们的研究结论。

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