Tight-Binding Molecular Dynamics Study of Liquid and Amorphous Carbon
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TllG]HT-IMNDIING MOLECULAR DYNAMIICS STUDY OF LffQUIID AND AMORPHOUS CARIBON C. Z. WANG, K. M. HO, AND C. T. CHAN Ames Laboratory and Department of Physics, Iowa State University, Ames, IA 50011
ABSTRACT Tight-binding molecular-dynamics simulations are performed to study the structure of liquid and amorphous carbon. Comparisons of our results with ab initio molecular dynamics (Car-Parrinello) results and experimental data show that the scheme has sufficient accuracy and efficiency for realistic simulation study of the structural properties of complex carbon systems.
ENTRODUCTION The ability of carbon to form strong chemical bonds under a variety of coordination numbers (from two to four) endows liquid and amorphous phases of carbon with many unique properties, and at the same time makes the system quite complex. Despite the considerable theoretical and experimental efforts [1-3] that have been devoted to the study of this fascinating materials, the understanding of their microscopic structures and physical properties is still far from being complete. In the last several years, there has been rapid progress in the study of these complex systems with realistic simulation techniques. Molecular dynamics(MD) and Monte Carlo(MC) studies have been performed using either Car-Parrinello scheme [3] or empirical classical potentials [4-6]. The Car-Parrinello method can describe the interatomic interactions accurately within the framework of ab initio density functional theory with local-density approximation (LDA). However, simulation for carbon systems demands a huge plane-wave basis set in order to obtain converged results, which makes the method rather expensive for extensive studies. The classical-potential simulations, on the other hand, can handle a larger number of atoms and longer simulation periods, but the interatomic interactions are ad hoc and less accurate. They also do not a priori yield information on the electronic structure of the system. Recently, we have developed a molecular dynamics scheme [7] which incorporates electronic structure effects into MD simulations through a tight-binding parametrization [8]. In this "tight-binding molecular dynamics (TBMD)" approach, the covalent bonding of the material enters the calculations in a natural way through the underlying electronic structure rather than through ad hoc N-body potentials. Unlike the Car-Parrinello approach which relies on expansion of the electronic wave functions by plane waves [3], the tightbinding electronic calculations require only a few atomic orbitals for each atom, allowing a larger number of atoms and longer simulation periods to be tackled within the present computer capabilities. Because of the above advantages, the TBMD scheme can bridge the gap between the Car-Parrinello approach and the simulations using classical potentials. Using a transferable tight-binding model for carbon recently developed in our group [9], we have performed extensive TBMD simulations for liquid and amorphous carbon. In this paper, we report on the results of t
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