Showing papers by "Paolo Giannozzi published in 2001"

Phonons and related crystal properties from density-functional perturbation theory

Abstract: This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.

Coverage-Dependent Adsorption of CH3S and (CH3S)2 on Au(111): a Density Functional Theory Study

Abstract: The origin of the superlattice present in the commensurate hexagonal structure of self-assembled monolayers of n-alkanethiols on gold and the question of whether the thiols are adsorbed onto the surface as dimers (disulfides) or monomers (thiolates) have been under debate for many years. Looking for a better understanding of the structural properties of these systems, we have performed a theoretical study of the molecular and dissociative adsorption of dimethyl disulfide on Au(111) as a function of coverage (0.25 e £ e 1), using gradient-corrected density functional (DFT) calculations with a slab geometry. For the dissociated state, our results indicate that the hcp hollow site is much less favorable than the fcc site. For the latter site, we find that, because of surface gold atom relaxation, the adsorption energy depends strongly on £, changing from 18 kcal/mol at £ ) 0.25 to 3 kcal/mol at £ ) 1. For the bridge site, instead, the adsorption energy is a weak function of £, and for all investigated coverages, this site is by far the most stable. According to our DFT approach, the adsorption of dimethyl disulfide is dissociative with a thermodynamic gain, at £ ) 1, of 13 kcal/mol with respect to the adsorbed molecular state. We also find, however, that the energy of c(4 2) structures containing at least two inequivalent CH 3S groups per unit cell (with a minimum S-S distance of 3.7 A) is, within the accuracy of our approach, indistinguishable from the pure (x3 x3) hexagonal structure. Our results suggest that the full solution of this thorny problem will require, also for the shortest chains, an estimate of the energetic contribution of dispersion forces that are not included in the DFT calculations.

Reconstruction of frozen-core all-electron orbitals from pseudo-orbitals

Abstract: We investigate the numerical feasibility of reconstructing frozen-core all-electron molecular orbitals from corresponding pseudo-orbitals. We perform density-functional calculations on simple atomic and molecular model systems using ultrasoft pseudopotentials to represent the atomic cores. We apply a transformation due to Blochl [Phys. Rev. B 50, 17953 (1994)] to each calculated pseudo-orbital to obtain a corresponding frozen-core all-electron molecular orbital. Our model systems include the reconstruction of the 5d orbital of a gold atom, and the occupied valence states of the TiO2 molecule. Comparison of the resulting all-electron orbitals to corresponding ones that were obtained from calculations in which the core electrons were explicitly included indicates that all-electron molecular orbital reconstruction is a feasible and useful operation in reproducing the correct behavior of molecular orbitals in the nuclear core regions.

Defect engineering in III–V ternary alloys: effects of strain and local charge on the formation of substitutional and interstitial native defects

Abstract: The effects of external and internal strains and of defect charges on the formation of vacancies, antisites and interstitials in GaAs and In0.5Ga0.5As have been investigated by first principles density functional methods. Present results show that strain and doping permit a defect engineering of III–V semiconductors. Specifically, they predict that doping may have major effects on the formation of antisites while vacancies may be favored only by extreme conditions of compressive strain. Interstitials may be moderately favored by doping and tensile strain.