Dielectric properties of condensed systems composed of fragments

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In order to predict the properties of solids and liquids composed of well-defined building blocks, it is important to determine the variation of the dielectric properties of the isolated ionic, molecular, or nano-scale constituents upon assembly. Hence the ability to compute dipole moments and polarizabilities of building blocks in condensed phases is critical. We propose a first-principles method to compute polarizabilities of sub-entities of solids and liquids, which accounts for multipolar interactions at all orders and is applicable to semiconductors and insulators. The method only requires the evaluation of induced fields in the condensed phase, with no need of multiple calculations for each constituent. [Pan and Galli (2018)].

CO2 in the Earth's mantle water

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The deep carbon cycle is of great importance in the Earth's carbon cycle, responsible for global warming and climate change. Deep inside the Earth, great amount of carbon may be transported by water. However, the forms of dissolved carbon in water under high pressure and high temperature conditions are not well known. Herein, by conducting extensive first-principles molecular dynamics simulations, we found the most of the dissolved carbon at 11 GPa and 1000 K is in the form of solvated carbonate and bicarbonate anions, contrary to popular geochemical models assuming that CO2 is the major carbon species present in water. Our results suggest that at such extreme conditions water transports carbon mostly through highly active ions, not CO2 molecules [Pan and Galli (2016)].

Extreme physics and chemistry of water

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Water is a major component of fluids in the Earth's mantle, where its properties are substantially different from those at ambient conditions. At the pressures and temperatures of the mantle, experiments on aqueous fluids are challenging, and several fundamental properties of water are poorly known; e.g., its dielectric constant has not been measured. This lack of knowledge of water dielectric properties has greatly limited our ability to model water-rock interactions and, in general, our understanding of aqueous fluids below the Earth's crust. Using ab initio molecular dynamics, we computed the dielectric constant of water under the conditions of the Earth's upper mantle, and we predicted the solubility products of carbonate minerals [Pan et al. (2013)]. The computed dielectric constant of water has been implemented in the latest Deep Earth Water model.

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Experimentally, it is not yet possible to measure absorption processes taking place in water and ice in diamond anvil cells. The band gap of diamond is smaller than that of water and ice, at least up to 30 GPa, and at high temperatures water becomes corrosive. We used ab initio molecular dynamics simulations and electronic structure calculations to show that both the refractive index and the electronic gap of water and ice increase with increasing pressure (up to 30 GPa), contrary to previous assumptions and contrary to the results of simple, widely used models. Subtle electronic effects, related to the nature of inter-band transitions and band edge localization under pressure, are responsible for this apparently anomalous behavior [Pan et al. (2014)].

Ice surface is proton ordered

projects

Ice Ih, the most common ice phase at ambient conditions, is a proton disordered crystal, obeying so called Bernal-Fowler-Pauling rules; however, we found the ice surface is significantly more proton ordered than the ice bulk [Pan et al. (2008)]. The ordered proton arrangements at the ice surface lead to a large variation of vacancy formation energies [Watkins et al. (2011)], and alter the adsorption preference of polar molecules on ice surfaces [Sun et al. (2012)].