Ilenkov I. Collective excitations in liquids with associates based on ab initio simulations

The thesis is devoted to the investigation of collective excitations in liquids with associates using ab initio molecular dynamics (AIMD) simulations and theoretical analysis within the framework of generalized hydrodynamics based on the approach of generalized collective modes. The systems of interest in this study include liquid antimony and hydrogen fluid under high-pressure. The estimation of eigenvalues and eigenvectors of the generalized hydrodynamic matrix, which is equivalent to solving the generalized Langevin equation in matrix form, provides a theoretical description of the time-dependent particle density and current correlation functions in liquids. This approach allows for the better insight in nature of collective excitations, as well as finding the behavior of collective modes beyond the hydrodynamic regime. An analysis of the structural and dynamical properties of liquid antimony was carried out. The radial distribution function of particles was obtained, characterized by a shoulder within the first coordination shell. Based on the time evolution of the distances between nearest neighbors, it was established that the shoulder is due to the preferred positions of quasi-bound atomic pairs. A five-variable thermo-viscoelastic model was applied that successfully reproduced the correlation functions obtained within the first-principles simulations. It was shown that one of the two observed propagating modes exhibits an extended flat region in the dispersion of collective excitations, which agrees with the AIMD results. Additionally, it was found that the collective excitations of quasi-bound pairs exhibit a flat dispersion for all studied wave-numbers k, that was not specific for the other modes. The collective dynamics of molecular hydrogen fluid was investigated. The five-variable thermo-viscoelastic model of generalized hydrodynamics was employed to reproduce four collective time correlation functions derived from AIMD simulations. It was established that using the imaginary part of the density response function and the correlation function of the time derivatives of the longitudinal current to determine the unknown static correlators and correlation times in the generalized hydrodynamic matrix accurately reproduces spectral properties of the longitudinal current correlation functions along with the dispersion of collective excitations. The long-wavelength asymptotes ~k^2 were obtained for the static correlators of the time derivatives of the longitudinal and transverse currents. The correct hydrodynamic behavior of the dispersion and damping of the complex eigenvalues, obtained from the generalized collective modes approach, was confirmed in the long-wavelength regime. An analysis of the structural and dynamical features of the hydrogen fluid in the pressure-induced transition region from the molecular to the atomic state was performed. The dispersion of collective excitations was determined for a range of densities at a temperature of 2500 K. The obtained density dependence of the adiabatic and high-frequency sound velocities exhibits a plateau in the transition region from molecular to atomic state. It was shown that the five-variable thermo-viscoelastic model accurately reproduces the time correlation functions obtained from AIMD simulations for purely atomic hydrogen fluid. The use of chemically reactive mixture model and the decomposition of the time correlation function contributions via the partial components of molecular and atomic subsystems was proposed.