Characterization of molecular-atomic transformation in fluid hydrogen under pressure via long-wavelength asymptote of charge density fluctuations

We report the observation of a pressure-induced non-monotonic behavior of the long-wavelength asymptote of total charge density fluctuations right in the region of molecular-to-atomic fluid transformation from ab-initio molecular dynamics (AIMD) simulations of fluid hydrogen. Screening in the total charge density composed of point protons and distribution of electronic density is studied within density functional theory (DFT) via total charge-charge static structure factor SQQDFT(k). We show from theory and AIMD that the long-wavelength behavior of SQQDFT(k → 0) ~ kη with η = 4 in neutral atomic and in fully metallic liquids, while η = 2 is in an ionic mixture with long-range Coulomb interaction. At high temperature (2500 K) we find a change in η with density around the region of molecular dissociation. At lower temperature (1000 K, below the critical point), we observe an η value different from 4 only in the pressure vs. density plateau region. This observation is consistent with a smooth molecular to atomic transformation, and implies existence of a semiconducting state with some unscreened protons responsible for long-range interaction.