  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.
