Amorphous carbon networks are used to test various levels of theoretical approaches to molecular dynamics simulations. The density-functional theory as implemented in the Car-Parrinello method, nonorthogonal tightbinding method, the environment-dependent interaction potential ͑EDIP͒, and the Brenner potential are compared directly in liquid quench simulations containing 125 atoms at four densities. We find that at low densities the predictions of the Brenner potential are in agreement with those from density-functional theory, while structures produced by nonorthogonal tight-binding method compare well with density-functional theory at all densities. The tight-binding method does, however, find a slightly lower sp 3 fraction at high densities and the presence of singly coordinated atoms at low densities. The frequency of three-membered rings are underpredicted by the tight-binding and EDIP methods due to an overestimate of strain energy relative to densityfunctional theory and experiment. Aside from the small rings, and a slight underestimate in sp 3 fraction at the highest densities, the EDIP simulations are in very good agreement with density-functional theory. The EDIP method is also used to quantify the statistical variability of liquid quenching, and comparisons with film growth simulations verify that liquid quenching is a good representation of bulk amorphous carbon.

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Maximally localized Wannier functions are the basis of a new technique for resolving ambiguous bonding issues for amorphous materials. Geometrical methods using the Wannier function representation provide an insightful chemical picture of local bonding and hybridization in disordered structures. Central to these methods is the notion of treating the Wannier function centres as a virtual atomic species with a well-defined degree of localization. Using Wannier function methods, we classify and quantify the types of bonding present in a sample of the ternary alloy hydrogenated amorphous silicon carbide, C22Si22H20. In addition to the bonding previously observed for this material, we see three-centre bonding and flipping bonds. We identify a cluster defect in our sample associated with these flipping bonds, and observe a temperature dependence of the bond flipping. This effect may be observable using temperature-dependent Raman spectroscopy.

A tight-binding simulation of the atom-by-atom deposition of amorphous carbon (a-C) at 100 eV incident energy is presented. More than 500 atoms were deposited. Chains are observed to form on the surface, some of which are sputtered. The good agreement with the experimental sputter frequency data and observation that all such clusters are linear provides strong support for the existence of these chains and the direct emission model of sputtering. The bulk of the grown film is a-C with a tetrahedral bonding fraction of 20%. Experiments have shown that at this incident energy of 100 eV, tetrahedral a-C is the preferred structural form rather than the a-C produced by this simulation. This discrepancy is attributed to the short range of the interatomic potential.

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