THE CONNECTIONS BETWEEN PROPERTIES AND LOCAL STRUCTURES OF LIQUID WATER AND SALT SOLUTIONS BASED ON FIRST-PRINCIPLES CALCULATIONS

Abstract

Liquid water is the most significant substance on earth. Its anomalous properties, stemming from its unique hydrogen bond (HB) network, contribute to its fundamental importance across a broad range of fields, including biochemistry, meteorology, and ecology. The HB network is governed by the interplay between covalent bonds, HBs, and van der Waals interactions, sensitive to even a slight alternation of these interaction strengths. Various scattering experiments and spectroscopy techniques have been developed to probe the effect of changes in HB network on the ensemble-averaged value of these properties. As complementary, ab initio Molecular dynamics (AIMD), combined with the machine learning techniques, can provide the information on atomic scale and help us identify the contributions from the water molecules in different local structures to these properties. This dissertation focuses on investigating the relationship between spectra probing the unoccupied states and local structures in NaCl solutions, as well as the connection between density and local structures in liquid water. The first part employs the GW-Bethe-Salpeter-Equation (GW-BSE) approach to reproduce theoretical XAS spectra of NaCl solutions and compare them with those of pure water. The introduction of ions disrupts the HB network, leading to the localization of excitons which causes the observable changes in the spectra. The second part delves into the investigation of the density anomaly of liquid water at atmospheric pressure. Three different molecular dynamics trajectories at each temperature from 290K to 390K with 10K interval are simulated using distinct machine-learning potential models. These models are trained on input data from density functional theory calculations based on different approximate exchange-correlation functionals, illustrating the impact of varying local structures on the density. Subsequently, Voronoi Polyhedra analysis is employed to establish a quantitative connection between the changes in density and the alternations of local structures in liquid water at different temperatures.