Generalized Chemical Model for Warm Dense Hydrogen: Ionization Potential Depression and Molecular Dissociation Shifts

Abstract

Warm dense matter (WDM) is a complex state, where quantum effects, thermal excitations, and strong interparticle correlations coexist. Understanding its microscopic composition and medium-induced modifications of atomic and molecular properties is essential for planetary modeling, fusion research, and high-energy-density experiments. We develop and apply the generalized chemical model to warm dense hydrogen in order to derive analytical estimates for two key quantities: the ionization potential depression (IPD) and the molecular dissociation energy shift. We show that the IPD depends explicitly on the ionization degree and cannot be captured by classical models such as those of Ecker-Kröll and Stewart-Pyatt. By consistently including neutral components, the present framework offers a more balanced description of plasma composition across different densities and coupling regimes. Furthermore, we demonstrate that the dissociation energy of hydrogen molecules undergoes a qualitative change: at weak-to-moderate coupling, molecular binding is weakened by interatomic interactions, while in the strong coupling regime, the intermolecular correlations dominate and enhance the stability of molecular species. These findings reveal the dual role of neutral correlations in warm dense hydrogen and provide analytical insights relevant to both astrophysical applications and laboratory experiments.