Showing papers by "Paolo Giannozzi published in 1992"

Towards Very Large-Scale Electronic-Structure Calculations

Abstract: We present a new approach to density functional theory, which does not require the calculation of Kohn-Sham orbitals. The computational workload required by our method—which is based on the calculation of selected elements of the Green's function—scales linearly with the volume of the system, thus opening the way to first-principles calculations for very large systems. Some of the problems which still hinder the achievement of this goal are discussed, and possible solutions are outlined. As an application, we calculate the charge density of a model silicon supercell containing 64 atoms slightly displaced at random from equilibrium.

Thirteen‐atom clusters: Equilibrium geometries, structural transformations, and trends in Na, Mg, Al, and Si

Abstract: We report the results of an extensive structural study of Na13, Mg13, Al13, and Si13 carried out with the Car–Parrinello method. Several and mostly unforeseen noncrystalline structures are discovered to characterize the low portion of the potential energy surface. Crystalline structures are shown either to correspond to high‐energy local minima or to be highly unstable. The low‐energy structural pattern appears to change significantly from one element to the other. Specific characteristics as well as trends are discussed.

Atomic and molecular hydrogen in gallium arsenide: A theoretical study.

Abstract: We present first-principles calculations of the properties of atomic and molecular hydrogen in pure bulk GaAs. Our results indicate that H penetrates into GaAs in atomic form. Inside GaAs, atomic H tends to form ${\mathrm{H}}_{2}$ molecules in tetrahedral sites, which are deep energy wells for ${\mathrm{H}}_{2}$. The ${\mathrm{H}}_{2}^{\mathrm{*}}$ defect, formed by one H in a bond-center site and one H in an adjacent tetrahedral position, has higher energy than ${\mathrm{H}}_{2}$ but lower-energy barriers for diffusion. Isolated H could be present as a metastable species. We compute the stable charge state of isolated H as a function of the Fermi energy. Our results suggest that H behaves as a negative-U defect. As a consequence, isolated H is expected to be present only as a charged species (positively charged in p-doped samples, negatively charged in undoped and n-doped samples). Our conclusions are compared with experimental results and with the results of calculations for H in other semiconductors. The main features of H in GaAs are quite similar to what has been found in Si.

Phonon Softening and High-Pressure Low-Symmetry Phases of Cesium Iodide

Abstract: The relative stability of various high-pressure phases of CsI is studied from first principles and analyzed using the Landau theory of phase transitions. We demonstrate that the cubic-to-orthorhombic transition recently observed to occur slightly below 20 GPa is driven by the softening of an acoustic phonon at the M point of the Brillouin zone. The coupling between this mode and anisotropic strain makes the transition slightly first order (with a volume variation of the order of 0.1%), and stabilizes the experimentally observed orthorhombic phase with respect to other competing symmetry-allowed structures

Silicon-hydrogen-acceptor complexes in crystalline silicon.

Abstract: An extensive investigation of the equilibrium sites of H in p-type silicon has been performed in order to clarify the inffluence of the specific impurity on the geometry of the silicon-hydrogen-acceptor complexes. Previous studies focused onto the B and Al cases have been extended to the isovalent acceptors Ga and In, making clear the relevance of the impurity atomic size. The on-axis BC site is shown to be a marginal equilibrium position, which evolves toward an off-axis position as soon as the acceptor size exceeds that of B. A novel H metastable site has been estimated, only in Si:In, at the AB-In site, thus suggesting a dependence of H equilibrium sites on the full chemistry of the impurity. These results account well for far-infrared measurements in Si:Al and Si:Ga, as well as for perturbed \ensuremath{\gamma}\ensuremath{\gamma} angular correlation results in Si:In.

Structure and Thermodynamics of SiGe Alloys from Computational Alchemy

Abstract: We present a new method for studying theoretically the structural properties of semiconductor alloys. The alloy is considered as a perturbation with respect to a periodic virtual crystal, and the relevant energies calculated by density-functional perturbation theory. We show that —up to second order in the perturbation— the energy of the alloy is equivalent to that of a lattice gas with only two-body interactions. The interaction constants of the lattice gas are particular linear response functions of the virtual crystal, which can be determined from first principles. Once the interaction constants have been calculated, the finite-temperature properties of the alloy can be studied rather inexpensively by MonteCarlo simulations on the lattice gas. As an application, we consider the case of SixGe1-x. A comparison with traditional self-consistent calculations for some simple ordered structures demonstrates that the accuracy of the perturbative approach is in this case of the same order as that of state-of-the-art density-functional calculations. Ignoring lattice relaxation, the range of the interactions is very short. Atomic relaxation renormalizes the interactions and makes them rather long range, propagating mainly along the bond chains. Monte Carlo simulations show that SixGe1-x is a model random alloy with a miscibility gap below≈ 170 K. The bond length distribution displays three well defined peaks whose positions depend on composition, but not on temperature. The resulting lattice parameter follows very closely Vegard’s law.