18 research outputs found
Machine Learning-Driven Structure Prediction for Iron Hydrides
We created a computational workflow to analyze the potential energy surface
(PES) of materials using machine-learned interatomic potentials in conjunction
with the minima hopping algorithm. We demonstrate this method by producing a
versatile machine-learned interatomic potential for iron hydride via a neural
network using an iterative training process to explore its energy landscape
under different pressures. To evaluate the accuracy and comprehend the
intricacies of the PES, we conducted comprehensive crystal structure
predictions using our neural network-based potential paired with the minima
hopping approach. The predictions spanned pressures ranging from ambient to 100
GPa. Our results reproduce the experimentally verified global minimum
structures such as \textit{dhcp}, \textit{hcp}, and \textit{fcc}, corroborating
previous findings. Furthermore, our in-depth exploration of the iron hydride
PES at different pressures has revealed complex alterations and stacking faults
in these phases, leading to the identification of several new low-enthalpy
structures. This investigation has not only confirmed the presence of regions
of established FeH configurations but has also highlighted the efficacy of
using data-driven, extensive structure prediction methods to uncover the
multifaceted PES of materials
Effective Static Approximation: A Fast and Reliable Tool for Warm Dense Matter Theory
We present an \emph{Effective Static Approximation} (ESA) to the local field
correction (LFC) of the electron gas that enables highly accurate calculations
of electronic properties like the dynamic structure factor , the
static structure factor , and the interaction energy . The ESA
combines the recent neural-net representation [\textit{J. Chem. Phys.}
\textbf{151}, 194104 (2019)] of the temperature dependent LFC in the exact
static limit with a consistent large wave-number limit obtained from Quantum
Monte-Carlo data of the on-top pair distribution function . It is suited
for a straightforward integration into existing codes. We demonstrate the
importance of the LFC for practical applications by re-evaluating the results
of the recent {X-ray Thomson scattering experiment on aluminum} by Sperling
\textit{et al.}~[\textit{Phys. Rev. Lett.} \textbf{115}, 115001 (2015)]. We
find that an accurate incorporation of electronic correlations {in terms of the
ESA} leads to a different prediction of the inelastic scattering spectrum than
obtained from state-of-the-art models like the Mermin approach or
linear-response time-dependent density functional theory. Furthermore, the ESA
scheme is particularly relevant for the development of advanced
exchange-correlation functionals in density functional theory
Probing Iron in Earth's Core With Molecular-Spin Dynamics
Dynamic compression of iron to Earth-core conditions is one of the few ways
to gather important elastic and transport properties needed to uncover key
mechanisms surrounding the geodynamo effect. Herein a new machine-learned
ab-initio derived molecular-spin dynamics (MSD) methodology with explicit
treatment for longitudinal spin-fluctuations is utilized to probe the dynamic
phase-diagram of iron. This framework uniquely enables an accurate resolution
of the phase-transition kinetics and Earth-core elastic properties, as
highlighted by compressional wave velocity and adiabatic bulk moduli
measurements. In addition, a unique coupling of MSD with time-dependent density
functional theory enables gauging electronic transport properties, critically
important for resolving geodynamo dynamics.Comment: 3 Figures in main document, 8 Figures in the supplemental informatio
Transferable Interatomic Potentials for Aluminum from Ambient Conditions to Warm Dense Matter
We present a study on the transport and materials properties of aluminum
spanning from ambient to warm dense matter conditions using a machine-learned
interatomic potential (ML-IAP). Prior research has utilized ML-IAPs to simulate
phenomena in warm dense matter, but these potentials have often been calibrated
for a narrow range of temperature and pressures. In contrast, we train a single
ML-IAP over a wide range of temperatures, using density functional theory
molecular dynamics (DFT-MD) data. Our approach overcomes computational
limitations of DFT-MD simulations, enabling us to study transport and materials
properties of matter at higher temperatures and longer time scales. We
demonstrate the ML-IAP transferability across a wide range of temperatures
using molecular-dynamics (MD) by examining the thermal conductivity, diffusion
coefficient, viscosity, sound velocity, and ion-ion structure factor of
aluminum up to about 60,000 K, where we find good agreement with previous
theoretical data
Electronic Density Response of Warm Dense Matter
Matter at extreme temperatures and pressures -- commonly known as warm dense
matter (WDM) in the literature -- is ubiquitous throughout our Universe and
occurs in a number of astrophysical objects such as giant planet interiors and
brown dwarfs. Moreover, WDM is very important for technological applications
such as inertial confinement fusion, and is realized in the laboratory using
different techniques. A particularly important property for the understanding
of WDM is given by its electronic density response to an external perturbation.
Such response properties are routinely probed in x-ray Thomson scattering
(XRTS) experiments, and, in addition, are central for the theoretical
description of WDM. In this work, we give an overview of a number of recent
developments in this field. To this end, we summarize the relevant theoretical
background, covering the regime of linear-response theory as well as nonlinear
effects, the fully dynamic response and its static, time-independent limit, and
the connection between density response properties and imaginary-time
correlation functions (ITCF). In addition, we introduce the most important
numerical simulation techniques including ab initio path integral Monte Carlo
(PIMC) simulations and different thermal density functional theory (DFT)
approaches. From a practical perspective, we present a variety of simulation
results for different density response properties, covering the archetypal
model of the uniform electron gas and realistic WDM systems such as hydrogen.
Moreover, we show how the concept of ITCFs can be used to infer the temperature
from XRTS measurements of arbitrarily complex systems without the need for any
models or approximations. Finally, we outline a strategy for future
developments based on the close interplay between simulations and experiments
Ab Initio Simulation of Warm Dense Matter: Combining Density Functional Theory and Linear Response Methods
Warm dense matter (WDM) is an extreme state of matter induced by extreme conditions and characterized as an intermediary state between (high-pressure) condensed matter and plasma. It has sparked a lot of attention in recent years as a result of current innovations in experiments and theoretical methods for modeling such complex systems. Such conditions naturally occur in astrophysical objects such as the interiors of the planets, and in white and brown dwarfs. WDM can be created in the laboratory via various methods such as laser compression, Z-pinches and heated diamond anvil cells.
This thesis describes the results obtained for many such systems across a range of conditions modeled using ab-initio simulation methods. The first testbed concerns the electronic structure and linear response of the carbon phases under high-pressure and warm dense matter conditions. The focus is on modeling inelastic x-ray scattering spectra across a range of conditions useful for the analysis and interpretation of x-ray Thomson scattering (XRTS) experiments. Another major goal is to improve the existing models to compute static properties such as the equation of state, density of states with the inclusion of highly accurate data from quantum Monte Carlo (QMC) simulations relevant at finite-temperatures. This approach improves the accuracy and is also computationally inexpensive compared to path integral Monte Carlo (PIMC) methods. Lastly, improvements in linear response theory relevant for XRTS are incorporated with the inclusion of local field corrections (LFC) and finite-temperature local field corrections (T-LFC) using data from QMC simulations
Data for "Dissociating the phononic, magnetic and electronic contributions to thermal conductivity: a computational study in α-iron"
This repository contains the data and script to generate the electronic component of the thermal conductivity in iron (alpha phase) relevant for the linked publication
Electrical Conductivity of Iron in Earth's Core from Microscopic Ohm's Law
Understanding the electronic transport properties of iron under high
temperatures and pressures is essential for constraining geophysical processes.
The difficulty of reliably measuring these properties under Earth-core
conditions calls for sophisticated theoretical methods that can support
diagnostics. We present results of the electrical conductivity within the
pressure and temperature ranges found in Earth's core from simulating
microscopic Ohm's law using time-dependent density functional theory. Our
predictions provide a new perspective on resolving discrepancies in recent
experiments