  The optical spectra of hydrogen at $\sim$500 GPa were studied theoretically
using a combination of \emph{ab initio} methods. Among the four most
competitive structures, i.e. C2/c-24, Cmca-12, Cmca-4, and I41/amd, only the
atomic phase I41/amd can provide satisfactory interpretations of the recent
experimental observation, and the electron-phonon interactions (EPIs) play a
crucial role. Anharmonic effects (AHEs) due to lattice vibration are
non-negligible but not sufficient to account for the experimentally observed
temperature dependence of the reflectance. The drop of the reflectance at 2 eV
is not caused by diamond's band gap reducing or interband plasmon, but very
likely by defects absorptions in diamond. These results provide theoretical
support for the recent experimental realization of metallic hydrogen. The
strong EPIs and the non-negligible AHEs also emphasize the necessity for
quantum treatments of both the electrons and the nuclei in future studies.
