Pseudoatom molecular dynamics

Abstract

A new approach to simulating warm and hot dense matter that combines density functional theory based calculations of the electronic structure to classical molecular dynamics simulations with pair interaction potentials is presented. The new method, which we call pseudoatom molecular dynamics (PAMD), can be applied to single or multi-component plasmas. It gives equation of state and selfdiffusion coefficients with an accuracy comparable to ab-initio simulations but is computationally much more efficient.PACS numbers: 51.20.+d, 51.30.+i, 52.25.Kn, 52.65.Yy The challenge of accurately modeling dense plasmas over a wide range of conditions represents an unsolved problem lying at the heart of many important phenomena such as inertial confinement fusion [1], exoplanets and white dwarfs [2,3]. The production of large scale and accurate tabulations of data such as equation of state and transport coefficients as a function of density and temperature is a formidable task, requiring a consistent quantum mechanical treatment of the many-electron problem together with a classical treatment of the nuclear motion. The atoms in the plasma may have bound states or be fully ionized, the electrons may be fully degenerate or approaching their classical limit. The nuclear fluid can range from weakly through to strongly coupled. A consistent, reliable and accurate treatment across all these physical regimes with an approach that remains computationally tractable remains as an open problem.Plasmas of interest are typically one to thousands of times solid density, and have temperatures from about 1eV (∼10kK) to thousands of eV. The difficulty of creating and controlling such plasmas in the laboratory explains the lack of experimental data to guide theoretical development, though ongoing campaigns at National Ignition Facility [4] and elsewhere (eg.[5]), and recent advances in X-ray scattering techniques [6] are beginning to shed light on this problem.From a simulations perspective, powerful and complex tools exist that can provide benchmark calculations. In the lower temperature regime (a few eV) one such tool is Kohn-Sham (KS) density functional theory molecular dynamics (DFT-MD) (eg. [7]). Electrons are treated quantum mechanically through KS-DFT and ions are propagated with classical MD. The simulations are very computationally expensive and this cost scales poorly with temperature, limiting the method to lower temperatures. In practice KS-DFT-MD also relies on a pseudopotential approximation, which reduces the computational overhead by limiting the number of actively modeled electrons, through an ad hoc modification of the electron- (r), n ext e (r) and n P A e (r), as described in the text. Also shown is the bound state (or ion) contribution (n ion e (r)) to n P A e (r) and the valence electron contribution n scr e (r). The double peak structure in n ion e (r) reflects the bound state shell structure in the aluminum ion, while the oscillations in the valence contribution n scr e (r) are the well known Friedel oscillations, whic...

Modified Enskog kinetic theory for strongly coupled plasmas

“…In a future publication, we will test the validity of the present theory against molecular dynamics simulations of realistic plasmas using the techniques recently presented in Ref. [23].…”

First-principles equation of state calculations of warm dense nitrogen

“…In an effort to make even greater gains in efficiency, DFT-based average-atom models make further approximations based on solving for the electronic properties of a single atom within the plasma 92 . Such models have been shown to predict the electronic structure of the isolated atoms well, and recent developments have begun a more consistent treatment of many-body systems 93 . OF-DFT and average-atom models are capable of simulating systems sizes up to a few hundred particles, but ultimately, along with DFT-MD, they are based on a ground-state framework, and it is important to develop more accurate, finite-temperature methods with fewer approximations, such as PIMC, to benchmark such cal- culations.…”

Warm Dense Matter Demonstrating Non-Drude Conductivity from Observations of Nonlinear Plasmon Damping

“…It is thus important to test recent theoretical studies [31][32][33][34][35][36][37][38] with accurate measurements of, e.g., the dynamic structure factor (DSF). The DSF is a central quantity to calculate transport properties [39].…”