Approximate-rate Approach to Pseudo-atom Molecular Dynamics Simulation of Hot Dense Plasmas

Molecular dynamics (MD) is widely used to simulate the behavior of plasmas, providing detailed descriptions of hydrodynamic, transport, as well as atomic-scale structure properties. The pseudo-atom molecular dynamics (PAMD) approach is a computationally efficient method that models the plasma as a collection of average atoms. In standard PAMD simulations, each atom’s electronic state is precomputed and held fixed throughout the time evolution. This frozen-state approximation breaks down in dense plasmas, where excited-state lifetimes are comparable to or shorter than the inverse plasma frequency. To address this issue, we developed a probabilistic framework that allows atomic states to evolve dynamically during the simulation. Our method maintains consistency with the local thermodynamic equilibrium (LTE) distribution while still permitting physically realistic fluctuations in excited-state populations. These fluctuations lead to noticeable differences in predictions of physical properties such as diffusion coefficients, microfield distributions, and spectral lineshapes.