Tight-binding Molecular Dynamics Study Research Articles

Tight-binding molecular dynamics study of vacancy-interstitial annihilation in silicon

We have studied the annihilation of a tetrahedral interstitial by a vacancy using molecular dynamics and a tight-binding model due to Goodwin-Skinner-Pettifor. At both 1473 and 1173 K, the annihilation process was dominated by the movement of the faster-diffusing vacancy. Once the vacancy moved to a nearest-neighbor position relative to the interstitial, annihilation was inevitable. As the initial distance between the T-interstitial and the vacancy increased, the number of pathways by which annihilation occurs increased accordingly. Fast and slow annihilation pathways were identified. The process was found to be highly stochastic in nature, requiring multiple simulations to provide adequate statistics. System size was not a factor in altering the results. The capture radius, within which annihilation is assured, was found to be between 5--6 \AA{} at 1473 K, although it is clear from the orientation dependence of the results that a spherically symmetric view of the capture radius is somewhat inappropriate. Limited investigation of annihilation between a (110) split interstitial and a vacancy confirmed the basic results of Tang et al. (Ref. 21) (using a Kwon tight-binding model) that the formation of a double-vacancy: double-interstitial configuration, dubbed a ``bond-defect'' by those authors, occurs in some but not all cases as part of the annihilation mechanism.

Tight binding molecular dynamics studies of boron assisted nanotube growth

In this paper we report a theoretical study of the effects of the presence of boron in growing carbon nanotubes. We employ a well established tight binding model to describe the interactions responsible for the energetics of these systems, combined with the molecular dynamics simulation technique and structural relaxation calculations. We find, in agreement with the previous theoretical/experimental work of Blase et al. [Phys. Rev. Lett. 83, 5078 (1999)], that boron favors (n,0) (zig-zag) tubular structures over (n,n) (arm-chair) ones by stabilizing the zig-zag edge. Furthermore, it is shown that boron has the effect of delaying the tube closure process, a fact which could explain the improved aspect ratio experimentally observed in nanotubes synthesized in the presence of boron. Our dynamical simulations lead us to propose a mechanism through which this extension of the closure time can be explained.

Pseudorotating Na 4 at finite temperatures: tight-binding molecular dynamics studies

We present the results of distance-dependent tight-binding molecular dynamics studies of Na 4 at finite temperatures, and especially discuss the pseudorotating pattern obtained at temperatures larger than 200 K. A pseudorotating barrier height of 0.023 eV is extracted.

A tight-binding molecular dynamics study of vibrational spectra of H-covered diamond (100) surfaces

Vibrational spectra of the hydrogen-terminated diamond (100) surfaces were investigated using the tight-binding molecular dynamics method. For the C(100)-(2×1):H surface the calculations allowed us to identify the symmetric and antisymmetric stretching CH vibrations at 2916 and 2934 cm −1, respectively, the former being more intense in the infrared spectrum than the latter. The simulations performed at a higher temperature reveal a strong anharmonic coupling of the CH vibrations. The CCH bending vibrations are found to be coupled to the lattice modes and to spread over a wide spectral interval. The vibrational spectrum of the dihydride C(100):2H surface shows wide bands, again due to strong anharmonic motion of atoms in a stressed surface structure. Although the calculated infrared spectrum is in a qualitative agreement with the HREELS spectrum of Lee and Apai (S.T. Lee and G. Apai, Phys. Rev. B 48 (1993) 2684), further computational studies are necessary to verify the results.

Tight binding molecular dynamics studies of the viscosity of liquid selenium

A tight binding technique is employed to study theinfluence of the size of the unit cell on molecular dynamics simulationsof liquid selenium at 870 K. Systems with 69, 138, 276 and 552 atomshave been studied within the Fermi operator expansion method. TheGreen-Kubo formula has been used to calculate the shear viscosity forall systems from the off-diagonal component of the stress tensor. Nodependence on the cell size could be found within the error bars definedby statistical accuracy. For the two smallest systems calculations up to60 ps have been performed where the viscosities become identical. Thesefindings indicate that the recently reported disagreement of DFTcalculations and experiments on the viscosity of liquid selenium at 870K must be attributed to a deficiency in the chosen exchange correlationfunctional rather than size effects.

Growth of amorphous semiconductors: tight-binding molecular dynamics study

In our computer simulation the preparation process was similar to the atom-by-atom deposition onto a substrate which is a standard procedure to prepare amorphous semicondutors. For the calculation of the interactions between carbon and silicon atoms, tight-binding potentials were used. The Newton equations of motion were solved by the Verlet algorithm.

Tight-binding molecular-dynamics study of a copper oxide Cu 4O

The empirical tight-binding molecular dynamic simulations of Cu 4O are performed. It is shown that the atomic and electronic structure can be calculated reasonably with this kind of TB model and repulsive potentials used in this paper. The calculated structures are in good agreement with the results obtained by atomic resolution electron microscope.

Tight-binding molecular-dynamics study of amorphous carbon deposits over silicon surfaces

We report in this paper a procedure to simulate the deposit of carbon diamondlike films over the Si(001) surface. We use the method of tight-binding molecular dynamics and well-known transferable C-C and Si-Si interactions. We propose for the Si-C interaction a weighted average of the single element interactions and test it by studying crystalline SiC, the molecule SiC, some small mixed clusters and the surfaces of $\ensuremath{\beta}$ SiC(001). Results of the deposition simulation for low carbon concentrations are presented, showing the formation of a thin mixed interface layer, and its structural and some electronic properties are described.

Tight-binding molecular dynamics study of transition metal carbide clusters

A minimal-parameters basis is used to obtain a transferable tight-binding parametrization of the Ni–C interactions applicable to binary Ni m C n clusters. The data base for fitting the parameters is obtained from ab initio results for small Ni m C n , n+ m⩽4, clusters obtained using the density functional method and the single, double and triple coupled-clusters method. The parametrization is incorporated into the tight-binding molecular dynamics scheme to study Ni m C n clusters of arbitrary sizes and the interaction of Ni with graphite. Our results are in very good agreement with experiment.

Temperature dependence of the phonon frequencies of molybdenum: a tight-binding molecular dynamics study

The phonon dispersion curves of molybdenum at zero temperature as well as at elevated temperatures are studied, using our recently developed semi-empirical tight-binding model. At zero temperature the phonon dispersion curves are obtained by frozen-phonon calculations as well as by a dynamical matrix approach. The phonon frequency shifts due to an increase in temperature are calculated by molecular dynamics (MD) simulations and the quasiharmonic contribution to the total frequency shift is determined. An alternative approach for calculating the frequency shifts without performing MD simulations is also presented.

Tight-binding molecular-dynamics studies of defects and disorder in covalently bonded materials

Tight-binding (TB) molecular dynamics (MD) has emerged as a powerful method for investigating the atomic-scale structure of materials – in particular the interplay between structural and electronic properties – bridging the gap between empirical methods which, while fast and efficient, lack transferability, and ab initio approaches which, because of excessive computational workload, suffer from limitations in size and run times. In this short review article, we examine several recent applications of TBMD in the area of defects in covalently bonded semiconductors and the amorphous phases of these materials.

A tight-binding molecular dynamics study of NimSin binary clusters

A transferable tight-binding parametrization of the Ni–Si interactions, from small binary NimSin clusters to bulk NiSi2, is presented within a minimal parameter basis. The data base for fitting the parameters is obtained from (i) ab initio results for the NiSi dimer obtained using the density functional method and the single, double and triple coupled clusters method, and (ii) band structure results for the bulk NiSi2. The parametrization is incorporated into the tight-binding molecular dynamics scheme to study medium size NimSin clusters. Our results are in very good agreement with experiment.

Tight binding molecular dynamics studies of Ga $_m$ As $_n$ and Al $_m$ As $_n$ clusters

The parametrized tight binding molecular dynamics method is used to study the structures and properties of small GaAs and AlAs clusters. Relative stabilities and ground state properties of GaAs clusters consisting of up to 4 atoms agree with those obtained in previous studies. Similar properties are found for small AlAs clusters. The present study of microclusters of both GaAs and AlAs revealed that the icosahedral structure is unstable for the binary clusters. Stability of such clusters increases monatomically as a function of As to Ga or As to Al ratio.

A tight-binding molecular dynamics study of phonon anharmonic effects in diamond and graphite

We study the temperature dependence of phonon frequency shifts and phonon linewidths in diamond and graphite using tight-binding molecular dynamics simulations. The calculation of one-phonon spectral intensities of several modes through velocity - velocity correlation functions allows a quantitative and nonperturbative study of these anharmonic effects. Our results for the zone-centre optical mode of diamond, for which experimental data are available, agree very well with first-order Raman-scattering measurements.

O( N) tight-binding molecular dynamics study of amorphous carbon

We have carried out molecular dynamics simulations of amorphous carbon using the recently developed density-matrix order N tight-binding molecular dynamics scheme. Structures and properties of amorphous carbon at three different regimes (i.e., porous, graphite-like, and diamond-like) have been studied. The simulation results demonstrate that the order N tight-binding molecular dynamics scheme is accurate and efficient for atomistic simulation of large and complex systems.

Magnetic properties of Ni and Fe clusters: a tight binding molecular dynamics study

We introduce an efficient approach which incorporates structural and magnetic ordering in transition metal clusters of sizes well beyond the range of ab initio techniques. The method combines the Hubbard model approximation with the tight binding molecular dynamics technique. Applications of the present scheme to Ni n ( n ⩽ 7), and Fe n ( n ⩽ 5) clusters show that while magnetic ordering does not affect the geometric ordering of Ni clusters, inclusion of magnetic effects strongly influences the structure of Fe clusters, forcing them to attain geometric configurations of higher symmetry than paramagnetic states.

Tight-binding molecular-dynamics study of density-optimized amorphous GaAs.

Tight-binding molecular-dynamics study of density-optimized amorphous GaAs.

Tight-binding molecular-dynamics study of point defects in GaAs.

Tight-binding molecular-dynamics simulations at 0 K have been performed in order to study the effect of defects (vacancies and antisites) in different states of charge on the electronic and structural properties of GaAs. Relaxations are fully included in the model, and for each defect we calculate the local atomic structure, the volume change upon relaxing, the formation energy (including chemical potential contributions), and the ionization levels. We find Ga vacancies to relax by an amount which is independent of the state of charge, consistent with positron lifetime measurements. Our calculations also predict Ga vacancies to exhibit a negative-U effect, and to assume a triply negative charge state for most values of the electron chemical potential. The relaxation of As vacancies, on the contrary, depends sensitively on the state of charge. The model confirms the two experimentally observed ionization levels for this defect, just below the conduction-band minimum. Likewise, Ga antisites exhibit large relaxations. In fact, in the neutral state, relaxation is so large that it leads to a ``broken-bond'' configuration, in excellent accord with the first-principles calculations of Zhang and Chadi [Phys. Rev. Lett. 64, 1789 (1990)]. This system also exhibits a negative-U effect, for values of the electron chemical potential near midgap. For As antisites, we find only a weak relaxation, independent of the charge. The model predicts the neutral state of the defect to be the ground state for values of the electron chemical potential near and above midgap, which supports the view that the EL2 defect is a neutral As antisite. Upon comparing the formation energies of the various defects we finally find that, for all values of the atomic chemical potentials, antisites are most likely to occur than vacancies.

Tight-binding molecular-dynamics study of transition-metal clusters.

An efficient semiempirical molecular-dynamics technique to treat interactions in transition metals is introduced. The method is based on the tight-binding scheme incorporating d delectrons. Optimized geometries are obtained using this scheme for ${\mathrm{Ni}}_{\mathit{n}}$ clusters for n up to 10. The expected general trends for the various cluster properties are well described by our results. Important qualitative differences from semiconductor clusters are observed. The technique appears to be promising in modeling interactions in alloys containing semiconductor and metal atoms.

Transferable tight-binding parameters: An application to Ni and Ni-Al alloys.

Two approaches for obtaining tight-binding parameters for metallic alloys are compared and contrasted with special regard for the application to large scale simulations such as may occur in tight-binding molecular dynamics studies.