Ionisation Calculations using Classical Molecular Dynamics

We present a simple model of dense hydrogen plasma in which pair potentials are employed between ions and neutral particles. Within the model, electrons are either bound in the hydrogen ground state or distributed in a uniform charge-neutralising background. By performing an ensemble of Molecular Dynamics (MD) simulations, the self-consistent ionisation state is computed through a free energy minimisation framework based on the technique of Thermodynamic Integration (TI). Particular attention is given to the transition between atomic gas and ionised plasma, where the effect of neutral interactions may be quantified beyond the hard-sphere paradigm. Furthermore, pressure ionisation is demonstrated to be a model-sensitive phenomena, depending on the treatment of both interaction and kinetic effects. The developed technique is general, and we discuss the applicability to a variety of MD models for partially ionised Warm Dense Matter (WDM).