Condensed phase effects on the electronic momentum distribution in the warm dense matter regime

We report ab initio calculations of the valence electron momentum distribution function n(p) and dynamic structure factor for warm dense Be at Mbar pressures. We observe an unexpected, strong reshaping of the Compton profile upon increasing density, even well before any significant core-wavefunction overlap or electride behavior occurs. We propose that this nonperturbative effect, which is due to a growing influence on n(p) of the orthogonalization of valence and core electron wave- functions with increasing density, is observable by inelastic x-ray scattering at x-ray free-electron lasers and large-scale laser-shock heating facilities, and may also be more generally important for thermodynamic properties of dense, partially-ionized plasmas.