Ionisation Calculations using Classical Molecular Dynamics

TL;DR

This paper introduces a self-consistent molecular dynamics approach using thermodynamic integration to calculate ionisation states in strongly interacting systems, with applications to partially ionised hydrogen plasma.

Contribution

It presents a novel method combining free energy minimisation with molecular dynamics for ionisation calculations, extending beyond traditional models.

Findings

Pressure ionisation observed with short-range neutral interactions.

Transition from atomic gas to ionised plasma analyzed.

Method applicable to various models of warm dense matter.

Abstract

By performing an ensemble of molecular dynamics simulations, the model-dependent ionisation state is computed for strongly interacting systems self-consistently. This is accomplished through a free energy minimisation framework based on the technique of thermodynamic integration. To illustrate the method, two simple models applicable to partially ionised hydrogen plasma are presented in which pair potentials are employed between ions and neutral particles. Within the models, electrons are either bound in the hydrogen ground state or distributed in a uniform charge-neutralising background. Particular attention is given to the transition between atomic gas and ionised plasma, where the effect of neutral interactions is explored beyond commonly used models in the chemical picture. Furthermore, pressure ionisation is observed when short range repulsion effects are included between neutrals. The developed technique is general, and we discuss the applicability to a variety of molecular dynamics models for partially ionised warm dense matter.