Tight binding simulations of disordered systems

Carbon allotropy allows the formation of a large variety of disordered structures whose properties depend on the density and on the preparation technique. Computer simulations, based on quantum mechanics, can reproduce the essential features of these systems. We first assess the reliability of the amorphous carbon structures generated by molecular dynamics simulations based on a semi-empirical Tight Binding hamiltonian. Then we attempt to understand of the path followed by diamond to reach the amorphous phase via a direct crystalline-to-amorphous transformation. Results concern the large variety of structures which can be obtained and a general adequacy of the semi-empirical Tight Binding hamiltonian to reproduce the essential features of amorphous carbon structure. Moreover, it is shown that the process of direct transformation from the crystalline into the amorphous phase occurs continuously. The formation of three-fold coordinated sites is not followed by an immediate site re-hybridization. When this process takes place, a large strain sites associated with the mis-coordinated sites is released.