Showing papers by "Roberto Car published in 1991"

Implementation of ultrasoft pseudopotentials in ab initio molecular dynamics.

Abstract: A scheme for the construction of ultrasoft separable pseudopotentials recently proposed by Vanderbilt is tested in the context of Car-Parrinello ab initio molecular-dynamics calculations on atoms and molecules. For the case of oxygen, the transferability of the pseudopotential is demonstrated by comparing the calculated properties of molecular ${\mathrm{O}}_{2}$ and ${\mathrm{O}}_{3}$ with those obtained from conventional approaches. Converged results are obtained using plane-wave-basis cutoffs of only \ensuremath{\sim}25 Ry. Forces can be calculated efficiently, and a molecular-dynamics simulation of molecular vibration is demonstrated.

Nitrogen and potential n-type dopants in diamond

Abstract: Potential n-type dopants in diamond are investigated via ab initio methods. The well-known distortions around the deep donor N are found to arise from the interaction of the N lone pair with a C dangling bond. P, Li, and Na are all shallow dopants, but their solubilities are muh too low for doping via in-diffusion. Li is a relatively fast diffuser, Na is stable up to moderate temperatures, while P should remain immobile even at high temperatures. Na, being an interstitial dopant, is particularly suitable for ion implantation, since there is no need to displace host atoms.

Binding and diffusion of a Si adatom on the Si(100) surface

Abstract: The binding sites for adsorption of a single Si atom on the reconstructed Si(100) surface are identified using first-principles total-energy calculations. We establish several saddle points for the migration of the adatom by mapping out the total energy as a function of its position on the surface. For the diffusion parallel to the dimer rows on the surface, we find an activation energy of 0.6 eV; for diffusion perpendicular to the rows, the activation energy is 1.0 eV. One-dimensional hopping motion of individual adatoms should be observable by scanning tunneling microscopy at moderately low temperatures.

Amorphous silicon studied by ab initio molecular dynamics: Preparation, structure, and properties

Abstract: We present a first-principles molecular-dynamics study of pure amorphous silicon obtained by simulated quench from the melt. A cooling rate of ${10}^{14}$ K/s is sufficient to recover a tetrahedral network starting from a well-equilibrated metallic liquid having average coordination larger than 6. Dramatic changes in physical properties are observed upon cooling. In particular, a gap forms in the electronic spectrum, indicating a metal-to-semiconductor transition. The as-quenched structure has average coordination very close to 4, but contains several coordination defects as well as a large fraction of distorted bonds. Subsequent annealing reduces the amount of strain and the number of defects present in our system. The average structural, dynamical, and electronic properties of our sample are in good agreement with the available experimental data. We report a detailed analysis of the structural relaxation processes accompanying annealing and compare our findings with recent experiments.

Structural, bonding, dynamical, and electronic properties of liquid silicon: An ab initio molecular-dynamics study.

Abstract: We report an extensive first-principles molecular-dynamics study of metallic liquid silicon. Our description of the local order is in excellent agreement with x-ray- and neutron-diffraction experiments. The difference in internal energy between the simulated liquid phase and the crystal agrees well with the experimental enthalpy of melting. Analysis of the valence-electronic-charge density shows persistence of some covalent bonds in the melt. These bonds give rise in the power spectrum of the system dynamics to a well-identifiable feature associated with stretching vibrations. Unlike the case in the crystal, in the liquid the covalent bonds are continuously forming and breaking in response to atomic motion. The majority of bonds are broken on average, leading to fast diffusion and to metallic behavior of the melt. The calculated electronic conductivity shows good agreement with available experimental data.

Structure, growth, and bonding nature of Mg clusters.

Abstract: The structure, growth, and bonding nature of ${\mathrm{Mg}}_{\mathit{n}}$ (n=2\char21{}13) clusters is studied using the density-functional molecular-dynamics method and the simulated annealing technique within the local-density approximation. We find a tetrahedron and a trigonal prism as two important constituents of the structure of Mg clusters to which atoms can be added by capping the faces. ${\mathrm{Mg}}_{13}$ is neither an icosahedron nor a cuboctahedron. The lowest-energy structure of ${\mathrm{Mg}}_{13}$ that we obtained from our simulated annealing can be considered as fusion of ${\mathrm{Mg}}_{9}$ and ${\mathrm{Mg}}_{4}$ clusters. This is nearly degenerate with the relaxed hcp structure and suggests the possibility of a path for transition to hcp structure for bigger clusters. We find ${\mathrm{Mg}}_{4}$ and ${\mathrm{Mg}}_{10}$ to be the magic clusters, which is in general agreement with the predictions of the jellium model of metal clusters. However, calculations of the charge densities, the p character, and the gap between the highest occupied and the lowest unoccupied states suggest the presence of mixed bonding character in Mg clusters and an oscillatory and slow convergence to bulk metallic behavior.

Structure of hydrogenated amorphous silicon from ab initio molecular dynamics

Abstract: We have generated a model of hydrogenated amorphous silicon by first-principles molecular dynamics. Our results are in good agreement with the available experimental data and provide new insight into the microscopic structure of this material. The calculation lends support to models in which monohydride complexes are prevalent, and indicates a strong tendency of hydrogen to form small clusters.

Comment on 'error cancellation in the molecular dynamics method for total energy calculations'

Abstract: In a recent paper (J. Phys. Cond. Matter vol.1, p.2199, 1989), M.C. Payne has analysed the method originally introduced by two of the authors to perform molecular dynamics simulations with interatomic forces derived directly from the electronic ground state (Phys. Rev. Lett. vol.55 p.2471, 1985). However the equations quoted in the paper by M.C. Payne do not correspond entirely to the original procedure. For this reason the authors have decided to write a joint comment in order to outline the differences and explain the consequences.

A density functional molecular dynamics study of the bonding and stability of Mgn clusters (n=2−13)

Abstract: A study of the structure and the bonding nature of Mg clusters having 2 to 13 atoms has been made using the density functional molecular dynamics method within the local density approximation. The calculated lowest energy structures can be described in terms of a tetrahedron and a trigonal prism. Mg4 and Mg10 are magic clusters and Mg13 is neither an icosahedron nor a cuboctahedron. The bonding nature varies from atom to atom in a cluster and the transition from weakly bonded dimer to bulk like metallic behaviour is oscillatory and slow.

Surface phonons and dipole activity of Si(111)2×1 from ab initio calculations

Abstract: Clarification d'une controverse sur l'attribution du pic de perte d'energie des electrons a environ 56 meV. La contribution dominante au pic de perte est anisotrope et provient d'une vibration optique longitudinale des chaines de surface. Cependant une faible contribution, mais non negligeable, provient de modes des soussurfaces polarises, perpendiculaires a la surface

Hydrogen in crystalline and amorphous silicon : a first principles molecular dynamics study

Abstract: Structural and dynamical properties of hydrogen in crystalline and amorphous silicon are analyzed by ab initio molecular dynamics simulations. In the crystalline case we focus mainly on the diffusion process of an isolated positively charged hydrogen impurity at high temperature, finding important dynamical effects. In the amorphous case we analyze the local order and the dynamical properties corresponding to an atomic hydrogen concentration of ~11%, typical of a device quality material. We find that hydrogen atoms form monohydride complexes and show interesting clustering effects. In both crystalline and amorphous cases, our results are in good agreement with available experimental data and give unique insight into the microscopic details of hydrogen incorporation in silicon.

Si(111): (2 × 1) reconstruction and surface phonons from ab-initio molecular dynamics

Abstract: We present results of ab-initio Molecular Dynamics simulations of the Si(111) surface. Pandey's 2 × 1 chain-geometry is obtained by simulated annealing, starting directly from the ideal bulk-terminated surface. The dynamical path followed during the reconstruction is described. The resulting 2 × 1 structure agrees well with Low Energy Electron Diffraction and Medium Energy Ion Scattering results. The phonon spectrum which is calculated ab-initio for the first time shows well defined surface modes at frequencies close to the measured values. The atomic motions and the dynamic effective charges of these modes are also analyzed.

Ab-Initio Calculation of Migration Energies of Adatoms on the Si (100) Surface

Abstract: The binding sites for adsorption of a single Si atom on the reconstructed Si (100) surface are identified using first-principles total energy calculations. By mapping out the total energy as a function of the position of the adatom on the surface, we establish several saddle points for migration over the surface. Similar calculations for P and Al adatoms establish the most important chemical trend.

Atomic Diffusion in Silicon

Abstract: We present a unified description of atomic diffusion processes in silicon, based on the results of parameter-free total-energy calculations. A consistent interpretation of both low- and high-temperature data is provided without the need to invoke a change in the nature of intrinsic defects as a function of temperature. The self-interstitial has negative-U properties and can migrate athermally via the Bourgoin-Corbett mechanism along several paths. Both vacancies and self-interstitials mediate self-diffusion with activation energies within the range of observed values. Both intrinsic defects also mediate the diffusion of impurities such as Al and P, but the detailed mechanisms are considerably more complex than those of self-diffusion.