Utilizing Wave Packet Molecular Dynamics to Study Warm Dense Matter
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Authors
Angermeier, William Alexander
Issue Date
2023
Type
Dissertation
Language
Keywords
Alternative Title
Abstract
Warm Dense Matter (WDM) has emerged as a focal point of scientific intrigue, owed to its substantial implications across a diverse array of physical systems, spanning from the interiors of exoplanets and the pursuit of sustainable energy via inertial confinement fusion to the atmospheric conditions of neutron stars. WDM refers to ionized fluids at the intersection of condensed matter physics, plasma physics, and dense liquids. This nexus position in thermodynamic parameter space makes it incredibly difficult and complex to model because the electrons exhibit quantum behavior, and the ions are strongly coupled. Most methods used to model WDM employ the Born-Oppenheimer approximation, also known as the adiabatic approximation, which assumes that the electrons and ions do not exchange energy. The validity of the adiabatic approximation in WDM is ultimately unknown, though people have tried to investigate this with varying degrees of success. This thesis constitutes an exploration, encapsulating three distinctive studies based on three published manuscripts, investigating the intricate behaviors of non-adiabatic effects on ion dynamics in WDM. Each study is rooted in the paradigm of wave packet molecular dynamics (WPMD). The WPMD method, known for its computational efficiency in exploring dynamic processes beyond the Born-Oppenheimer (BO) approximation, relies on implementing simplified approximations for computational expediency.The first study concentrates on elucidating the behavior of aluminum through non-adiabatic simulations, utilizing the electron-force field (eFF) variant of wave-packet molecular dynamics. Validation efforts centered on comparing the ion-ion structure factor with density functional theory (DFT) across diverse temperatures and densities within the WDM regime, specifically targeting conditions of 3.5 eV and 5.2 g/cm$^3$. The comprehensive analysis of the dynamic structure factor and dispersion relation, employing both adiabatic and non-adiabatic techniques, revealed a notable alignment between the dispersion relation obtained through eFF and the robust Kohn-Sham DFT. The second study rigorously examined three prevalent approximations within WPMD-restricted basis sets, exchange approximations, and correlation omissions-focusing on their impact on atomic and molecular hydrogen and dense hydrogen plasma. Integrating a two-Gaussian basis with a semi-empirical correction derived from the valence-bond wave function demonstrated a significant improvement, enabling precise scaling of this correction to align with experimental pressures. These findings underscore the pivotal role of semi-empirical scaling parameters in rectifying primary WPMD approximations, emphasizing their role in enhancing accuracy and unveiling underlying physics insights. The third study investigates a novel model-independent method introduced to assess the impact of non-adiabatic electron-ion interactions on transport properties in WDM. The primary focus of this inquiry is how non-adiabatic electron-ion interactions influence equilibrium ion dynamics in warm, dense hydrogen. This unique method overcame previous difficulties of comparing adiabatic and non-adiabatic simulation, which generally suffered from the underlying approximations made by each model, by using a non-adiabatic simulation of electrons and ions to obtain a forced matched ion potential with the same level of approximations. The forced matched potential was implemented through classical molecular dynamics to serve as the adiabatic simulation method, as the system did not directly include electron-ion interactions. This method, applied across diverse conditions in warm, dense hydrogen, concluded that non-adiabatic effects exert minimal influence on the ion self-diffusion coefficient.This comprehensive approach to evaluating the necessity of non-adiabatic simulations offers critical insights in a research landscape marked by limited experimental data on WDM. These findings substantially validate the adiabatic approximation for simulations of extreme states of matter, significantly advancing our understanding of WDM phenomena.