Microscopic structure of tetrahedral amorphous carbon

Simulations are reported of a highly tetrahedral amorphous carbon network at a density of 3 g/cc using Car-Parrinello first principles molecular dynamics. The simulated structure consists of 65% fourfold and 35% threefold coordinated carbon sites, in good agreement with experiment. The structure is also in good agreement with recent neutron diffraction data. An unexpected observation was the presence of small carbon rings in the structure containing as few as three atoms. These carbon triangles and quadrilaterals, which resemble the organic compounds cyclopropane and cyclobutane, give the network a topology unique among tetrahedrally bonded amorphous materials.