A transferable tight-binding model for silicon is found by fitting the energies of silicon in various bulk crystal structures and examining functional parametrizations of the tight-binding forms. The model has short-range radial forms similar to the tight-binding Hamiltonian of Goodwin, Skinner, and Pettifor but can be utilized in molecular dynamics with a fixed radial cutoff for all structural configurations. In addition to a very good fit to the energy of Si in different bulk crystal structures the model describes very well the elastic constants, defect-formation energies for vacancies and interstitials in crystalline silicon, the melting of Si, and short-range order in liquid silicon. Results for phonon frequencies and Griineisen constants in c-Si are also presented.

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We report electrical conductivities for a hydrogen plasma at temperatures between a few tenths to a few tens of electron volts and densities ranging from 0.3 to 3 g/cm 3 . The ac conductivities were determined within the Kubo-Greenwood formulation based on eigenstates from a finite-temperature density functional calculation at selected time steps along a lengthy molecular-dynamics ͑MD͒ simulation trajectory. Density functional, tightbinding, and effective pair potentials were employed in the MD simulations for samples of 50 to 250 atoms within a periodically replicated reference cell. We compare with other techniques and discuss trends with density and temperature. Good agreement results at the higher temperatures and densities with generalized Ziman forms. ͓S1063-651X͑96͒06109-0͔

Voids of various sizes have been introduced into amorphous-silicon models that were generated with molecular-dynamics simulations. The presence of the voids leads to a rapidly increasing strucol ture factor for wave vectors below 1 A that is similar to the intense small-angle scattering observed in experiments. The voids cause only small changes in the vibrational densities of states. The presence of voids decreases the local strain in the a-Si networks, leading to a substantial reduction in the number of five-coordinated defect sites and a somewhat lower bond-angle strain from the a-Si model without voids. By comparing the properties of voids in crystalline and amorphous structures, we find the effect of decreasing the bond-angle disorder is to make the TO peak of the densities of states narrower and stronger, in agreement with Raman measurements.

Both amorphous and epitaxial crystalline Si films have been grown by deposition of Si-atom clusters on a Si(111) substrate with molecular-dynamics simulations utilizing twoand three-body interatomic Si potentials. The amorphous films were produced with deposition conditions that led to low surface diffusion. The a-Si films displayed voids, a 15-28% lower density than c-Si, and coordination defects consisting only of undercoordinated atoms with no fivefold-coordinated atoms, in contrast to melt-quenched a-Si models. The epitaxial Si(111)growth was achieved under conditions of high surface diffusion consisting of a large initial cluster velocity and moderate substrate temperatures.