X-ray Thomson scattering spectra from DFT-MD simulations based on a modified Chihara formula

TL;DR

This paper compares advanced computational methods for calculating x-ray Thomson scattering spectra from DFT-MD simulations, highlighting the strengths and limitations of a modified Chihara formula and LR-TDDFT in different density regimes.

Contribution

It introduces a comparative analysis of dielectric function-based and LR-TDDFT approaches for scattering spectra, revealing their applicability and limitations in high-pressure conditions.

Findings

Excellent agreement between methods for ambient-density beryllium

Breakdown of dielectric approach in highly compressed, pressure-ionized matter

LR-TDDFT reanalysis shows deviations from traditional models at small angles

Abstract

We study state-of-the-art approaches for calculating x-ray Thomson scattering spectra from density functional theory molecular dynamics (DFT-MD) simulations based on a modified Chihara formula that expresses the inelastic contribution in terms of the dielectric function. We compare the electronic dynamic structure factor computed from the Mermin dielectric function using an ab initio electron-ion collision frequency to computations using a linear response time dependent density functional theory (LR-TDDFT) framework for hydrogen and beryllium and investigate the dispersion of free-free and bound-free contributions to the scattering signal. A separate treatment of these contributions in the Mermin dielectric function shows excellent agreement with LR-TDDFT results for ambient-density beryllium, but breaks down for highly compressed matter where the bound states become pressure ionized. LR-TDDFT is used to reanalyze x-ray Thomson scattering experiments on beryllium demonstrating strong deviations from the plasma conditions inferred with traditional analytic models at small scattering angles.