  The electronic structure and dielectric properties of the diamond, body
centered cubic diamond (bc8), and hexagonal diamond (lonsdaleite) phases of
carbon are computed using density functional theory and many-body perturbation
theory with the emphasis on the excitonic picture of the solid phases relevant
in the regimes of high-pressure physics and warm dense matter. We also discuss
the capabilities of reproducing the inelastic x-ray scattering spectra in
comparison with the existing models in light of recent x-ray scattering
experiments on carbon and carbon bearing materials in the Megabar range.
