Showing papers by "Roberto Car published in 1984"

Microscopic theory of atomic diffusion mechanisms in silicon

Abstract: We report self-consistent Green's-function total-energy calculations which provide, for the first time, a firm theoretical framework for understanding the microscopic mechanisms of atomic diffusion in Si. We find that the self-interstitial has negative-$U$ properties, roughly the same formation energy at several sites, small migration barriers, and charge-state instabilities that allow athermal migration along several paths. We also find that both vacancies and interstitials mediate self-diffusion and reconcile contrasting low- and high-temperature data.

Equilibrium Geometries and Electronic Structures of Small Sodium Clusters

Abstract: We report self-consistent local-spin-density calculations which provide, for the first time, the equilibrium geometries of the sodium clusters ${\mathrm{Na}}_{n}$ and ${{\mathrm{Na}}_{n}}^{+}$ with $nl~8$ and $n=13$, without making any a priori assumptions. Our results are in excellent agreement with recently obtained photoionization appearance potentials and electron-spin-resonance spectra. We find rapid formation of the metallic bond with a dominance of closely packed structures in two and three dimensions, and propose a simple model to account for it.

Calculation of Cluster Geometries with the Help of Hellmann-Feynman Forces

Abstract: The equilibrium geometries of Nan clusters (n ⩽ 7) are calculated by letting randomly generated clusters relax under the action of the Hellmann-Feynman forces. We find that clusters with five atoms or less have a planar structure whereas larger clusters have closely packed three-dimensional geometries. The calculated adiabatic ionization potentials are in good agreement with the experimental appearance potentials.

Microscopic theory of atomic diffusion mechanisms in silicon

Abstract: Self-interstitials in Si are known to migrate athermally at very low temperatures (−4 K). In contrast, at hightemperatures (1100–1600 K), self-diffusion has an activation energy of −5 eV. We describe results of self-consistent Green's-function total energy calculations which, for the first time, provide detailed microscopic understanding of the mechanisms underlying these phenomena and reconcile the contrasting low- and high-temperature data.

Forces in pseudopotential molecular calculations

Abstract: The forces derived from the Hellmann–Feynman theory, are calculated for a molecule, using the local density approximation and within the pseudopotential scheme. We show that, in contrast to the general result of all‐electrons calculations, the pseudopotential scheme allows us to obtain accurate and reliable forces with a small number of basis functions. Fast convergence in the basis set is crucial for calculations of large molecules. We apply our method to the Na2 molecule and discuss the convergence rate in detail.