| An understanding of aqueous ionic solutions is essential in developing a mechanistic view of many biological systems, industrial processes and others. Examples of important applications include biological processes such as blood pressure control, and industrial processes such as water desalination. Applying molecular dynamics simulations to describe aqueous ionic solutions could help to unveil the properties of aqueous solutions from their detailed molecular structures. Although it was among the first and simplest systems investigated by molecular dynamics, dynamical properties, such as water diffusivity, could not be described even qualitatively correctly. The origin of this problem is the inaccuracy of the underlying force field used in molecular dynamics. The force field---the analytical formulas describing interactions between molecules---could be improved by two approaches: Adding more physical terms, such as polarizable effects and charge transfer effects, is one way to make a better force field. The other approach is first principles molecular dynamics where the electronic structure calculation serves as the underlying force field directly instead of analytical forms of interactions.;In my Ph.D. work, I investigated aqueous ionic solution systems by first principles molecular dynamics and advanced classical molecular dynamics with polarizable effects and charge transfer effects included. A single ion in the liquid water system is an ideal system to investigate the effect of the ion on the nearby water molecules. I used first principles molecular dynamics as the benchmark to test other analytical force fields in such systems. Charge transfer effects were found to be essential in describing water diffusion dynamics correctly. This finding was then applied to more realistic systems of concentrated aqueous ionic solutions. With charge transfer effects included, the concentration-dependent water diffusivity was observed to be in line with the experimental data both qualitatively and quantitatively. Based on these two works, I concluded that charge transfer is important in describing water diffusivity in aqueous ionic solutions. In the above two works, first principles molecular dynamics was used as the benchmark method. Nevertheless, despite its popularity, first principles molecular dynamics is not guaranteed to be accurate. Instead, its accuracy depends strongly on the underlying electronic structure theory. Specifically, the accuracy of the most commonly used density functional theory depends on the exchange correlation functional. I applied two of the most recently-developed advanced exchange correlation functionals to study the potential of mean force for NaCl ion-separation in aqueous ionic solutions. I reported the most accurate prediction to date for the potential of mean force. How the underlying exchange correlation functionals impact the electronic structure, especially charge transfer effects, was also studied in this work. I recommended more applications of these two advanced functionals in the research of aqueous ionic solutions. Because of the growing appreciation of the importance of charge transfer, I investigated how to include charge transfer effects in a more succinct way for classical molecular dynamics. With a recently-developed theory of atom condensed Kohn-Sham to second order, I developed a model of liquid water and solutions to correctly describe charge transfer and polarizable effects. |
