Microscopic structure of hydrogenated amorphous carbon

We have investigated hydrogenated amorphous carbon at a density (2.6 g/${\mathrm{cm}}^3$) and H content (16%) corresponding to hard films, by means of first-principles molecular dynamics. Our computer-generated sample consists of about 55% fourfold- 40% threefold- and a small fraction of twofold-coordinated carbon sites, in agreement with recent measurements. All H atoms are bonded to C; a great majority of them is bonded to C atoms linked to three other carbons, giving rise to ${\mathrm{C}}_4$-H units, and thus promoting ${\mathit{sp}}^3$ bonding. Only monohydrated C sites are found in our simulation. Our results for total and partial correlation functions compare well with those obtained in neutron-scattering experiments.