Quantum Simulation Of Structure And Dynamical Properties Of Dense Light-atom Matter

The study on structure and dynamical properties of matter in extreme states is one of the central subjects of scientific disciplines. The light-atom matter such as hydrogen and water are the main components of the giant planets in solar system and recently constantly discovered planets. The isotopes of hydrogen(deuterium and tritium) are the fuel of inertial confinement fusion. The knowledge of structure and dynamical properties of light-atom matter is not only of fundamental significance in physics, but also crucial to high energy density physics involving astrophysics, geophysics, planetary science and inertial confinement fusion. Here we investigate the nuclear quantum effects(NQEs) on structure and transport properties of warm dense hydrogen as well as the phase transition mechanism and K-edge x-ray absorption spectra of high-pressure ice VII, VIII and X based on density functional theory and path-integral molecular dynamics(PIMD). Meanwhile,the structure and polarization properties of water, which is a typical hydrogen-bond system, under warm dense conditions are investigated using ab initio molecular dynamics.Furthermore, in order to understand the properties of water, the structure and vibrational properties of small water cluster are also studied.Firstly, within the framework of PIMD, we improve the centroid molecular dynamics in order to describe the nuclear quantum dynamics more reasonably. The calculation method for electronic properties based on density functional theory is combined with the PIMD approach, which realizes the "all-quantum simulation" for material structure and dynamical properties. Using this approach, NQEs on structure and transport properties of warm dense hydrogen at a temperature range from 0.1 eV to 1 eV and a density range from 10 g/cm3 to 100 g/cm3 are investigated using ab initio PIMD simulations. The results show that the NQEs broaden the radial distribution function strongly. The quantum nature of nuclei decrease the ionic elastic collision cross section, therefore the ionic self-diffusion coefficient is strongly enhanced and the viscosity coefficient is decreased accordingly. NQEs make more localization electrons surrounding ions, thereby heavily suppressing the electrical and thermal conductivity while enhancing the optical absorption. The Stokes-Einstein relation of self-diffusion and viscosity is not valid in strong coupling regime. The Lorenz number in Wiedemann-Franz law of electrical and thermal conductivity deviates slightly from the value of the degenerate limit.Secondly, temperature-dependent phase transition and oxygen K-edge x-ray absorption near-edge spectra(XANES) of ice VIII, VII and X at temperatures less than 300 K and pressures lower than 110 GPa are investigated using ab initio path-integral molecular dynamics simulations. Nuclear quantum tunneling plays a crucial role in the phase transitions between ice VII and VIII. Proton tunneling assists the proton-ordered ice VIII to transform into proton-disordered ice VII. NQEs have significant impacts on the phase transition temperature and pressure among three phases. Quantum simulations greatly improve the phase diagram and almost fill the gap between theoretical calculation and experiments. The calculated oxygen K edge of ice VIII at 2.2 GPa is in good agreement with experimental result. The XANES is sensitive to the proton order-disorder transition between ice VIII and VII, which can be used to diagnose the structure and dynamical properties of high-pressure ice.Thirdly, the structure and polarization properties of warm dense water at a density range from 1.0 g/cm3 to 2.2 g/cm3 and temperature range from 300 K to 2800 K are investigated using ab initio molecular dynamics simulations. Along the isochore of 1.0 g/cm3,the hydrogen-bond network of liquid water above 800 K collapses. The two shells of nearest neighbor and second-nearest neighbor of oxygen atoms merge into a single nearest neighbor shell. Along the isotherm of 1800 K, the transition from the liquid state to an amorphous superionic phase occurs at 2.0 g/cm3(32.9 GPa). With increasing temperature, the average dipole moment of water molecules is decreased, while it is contrarily increased with increasing pressure. Both higher temperature and pressure broaden the distribution of dipole moment of water molecules due to the enhanced intramolecular charge fluctuations.Fourthly, the structure and vibrational spectra of(H2O)n(n = 2–5) clusters have been studied using ab initio molecular dynamics simulations. The ground-state structures of(H2O)n(n = 2–5) are determined through comparing the calculated vibrational spectra with experimental results. The structure and vibrational mode are changed with increasing temperature. With increasing temperature, different frequency bands exhibit different behavior. shifting to red or blue even vanishing occur at different bands. The changes of vibrational spectra of water clusters could be used to diagnose the structure and dynamical properties.