Spectral reflectance of GaAs from infrared (IR) to ultraviolet (UV) bands is calculated from first principles. We first calculate the spectral dielectric function which is determined by the response of GaAs to the external electromagnetic field. Two mechanisms exist for different wavelengths, namely, phonon response in the far-IR region and the electronic absorption in the near-IR to UV region. With plane-wave pseudopotential method, we determined the dielectric function of GaAs with the the initial structure as the only input. For the far-IR region, phonon calculations are carried out. By analyzing the phonon modes, low-frequency dielectric constant is calculated using the...