We compute the linear optical properties of different reconstructions of th
e clean and hydrogenated Si(100) surface within DFT-LDA, using norm-conserv
ing pseudopotentials. The equilibrium atomic geometries of the surfaces, de
termined from self-consistent total-energy calculations within the Car-Parr
inello scheme, strongly influence reflectance anisotropy spectra, showing d
ifferences between the p(2 x 2) and c(4 x 2) reconstructions. The different
ial reflectivity spectrum for the c(4x2) reconstruction shows a positive pe
ak at (h) over bar omega<1 eV, in agreement with experimental results. [S01
63-1829(99)03928-4].