Ab initio Density Response and Local Field Factor of Warm Dense Hydrogen

We present quasi-exact ab initio path integral Monte Carlo (PIMC) results for the partial static density responses and local field factors of hydrogen in the warm dense matter regime, from solid density conditions to the strongly compressed case. The full dynamic treatment of electrons and protons on the same footing allows us to rigorously quantify both electronic and ionic exchange--correlation effects in the system, and to compare with earlier incomplete models such as the archetypal uniform electron gas [Phys. Rev. Lett. 125, 235001 (2020)] or electrons in a fixed ion snapshot potential [Phys. Rev. Lett. 129, 066402 (2022)] that do not take into account the interplay between the two constituents. The full electronic density response is highly sensitive to electronic localization around the ions, and our results constitute unambiguous predictions for upcoming X-ray Thomson scattering (XRTS) experiments with hydrogen jets and fusion plasmas. All PIMC results are made freely available and can directly be used for a gamut of applications, including inertial confinement fusion calculations and the modelling of dense astrophysical objects. Moreover, they constitute invaluable benchmark data for approximate but computationally less demanding approaches such as density functional theory or PIMC within the fixed-node approximation.