Ab initio simulations of tetrahedral amorphous carbon

Simulations are reported of a highly tetrahedral amorphous carbon network at a density of 2.9 g/cm(3) using Car-Parrinello first principles molecular dynamics. The network was generated by cooling a liquid carbon sample of 64 atoms to form an amorphous structure. The simulated structure is in good agreement with recent neutron diffraction data and contains 65% fourfold- and 35% threefold-coordinated carbon sites. Three- and four-membered rings are present in the structure and give the network an unusual topology. Evidence from neutron diffraction and organic chemistry is shown to support the existence of these ring structures, which resemble the carbon compounds cyclopropane and cyclobutane. Two additional networks were produced using different cooling rates. It was found that the number of fourfold sites decreased as the cooling time increased. This result has implications for the interpretation of experiments and models of how tetrahedral amorphous carbon is formed.