S. de Gironcoli

Advanced capabilities for materials modelling with Quantum ESPRESSO

Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudo-potential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement theirs ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.Quantum EXPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the-art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudopotential and projector-augmented-wave approaches. Quantum EXPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement their ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

Temperature dependent surface relaxations of Ag(111)

The temperature dependent surface relaxation of Ag(111) is calculated by density-functional theory. At a given temperature, the equilibrium geometry is determined by minimizing the Helmholtz free energy within the quasiharmonic approximation. To this end, phonon dispersions all over the Brillouin zone are determined from density-functional perturbation theory. We find that the top-layer relaxation of Ag(111) changes from an inward contraction (-0.8 %) to an outward expansion (+6.3%) as the temperature increases from T=0 K to 1150 K, in agreement with experimental findings. Also the calculated surface phonon dispersion curves at room temperature are in good agreement with helium scattering measurements. The mechanism driving this surface expansion is analyzed.

Ab initio phonon calculations in solids

Abstract We present some applications of a first-principles approach to the study of the vibrational properties of crystals. The ab initio lattice dynamics is studied by means of a perturbative approach to the density-functional theory. The validity of this method is investigated performing the calculation of the phonon frequencies of crystals with different structure and bonding properties. The results obtained are in excellent agreement with the available experimental data.

Highly under-coordinated atoms at Rh surfaces: interplay of strain and coordination effects on core level shift

The electronic structure of highly under-coordinated Rh atoms, namely adatoms and ad-dimers, on homo-metallic surfaces has been probed by combining high-energy resolution core level photoelectron spectroscopy and density functional theory calculations. The Rh3d5/2 core level shifts are shown to be proportional to the number of Rh nearest-neighbours (n = 3, 4 and 5). A more refined analysis shows that the energy position of the different core level components is correlated with the calculated changes of the individual inter-atomic bond length and to the energy changes of the d-band centre, which is known to be a reliable descriptor of local chemical reactivity.

The reconstruction of Rh(001) upon oxygen adsorption

Abstract We report on a first-principles study of the structure of O/Rh(001) at half a monolayer of oxygen coverage, performed using density functional theory. We find that oxygen atoms sit at the center of the black squares in a chess-board c(2×2) pattern. This structure is unstable against a rhomboid distortion of the black squares, which shortens the distance between an O atom and two of the four neighboring Rh atoms, while lengthening the distance with respect to the other two. We actually find that the surface energy is further lowered by allowing the O atom to get off the short diagonal of the rhombus thus formed. We predict that the latter distortion is associated with an order–disorder transition, occurring below room temperature. The above rhomboid distortion of the square lattice may be seen as a rotation of the empty white squares. Our findings are at variance with recent claims based on STM images, according to which it is instead the black squares which would rotate. We argue that these images are indeed compatible with our predicted reconstruction pattern.

The mechanism for the 3×3 distortion of Sn/Ge(111)

Abstract We show that two distinct 3×3 ground states — one non-magnetic, metallic and distorted; the other magnetic, semi-metallic (or insulating) and undistorted — compete in α-phase adsorbates on semiconductor (111) surfaces. In Sn/Ge(111), local (spin) density approximation (LSDA) and GGA calculations indicate, in agreement with experiment, that the distorted metallic ground state prevails. The reason for the stability of this state is analysed, and traced to a sort of bond density wave, specifically a modulation of the antibonding state filling between the adatom and a GeGe bond directly underneath.

DIPOLE-QUADRUPOLE INTERACTIONS AND THE NATURE OF PHASE III OF COMPRESSED HYDROGEN

A new class of strongly infrared active structures is identified for phase III of compressed molecular H2 by constant-pressure ab initio molecular dynamics and density-functional perturbation calculations. These are planar quadrupolar structures obtained as a distortion of low-pressure quadrupolar phases, after they become unstable at about 150 GPa due to a zone-boundary soft phonon. The nature of the II-III transition and the origin of the IR activity are rationalized by means of simple electrostatics, as the onset of a stabilizing dipole-quadrupole interaction.

Electron-Phonon Interaction at the Be(0001) Surface

We present a first principle study of the electron-phonon (e-p) interaction at the Be(0001) surface. The real and imaginary parts of the e-p self-energy (Sigma) are calculated for the Gamma; surface state in the binding energy range from the Gamma; point to the Fermi level. Our calculation shows an overall good agreement with several photoemission data measured at high and low temperatures. Additionally, we show that the energy derivative of Re Sigma presents a strong temperature and energy variation close to E(F), making it difficult to measure its value just at E(F).

Structural and Electronic Properties of a Wide-Gap Quaternary Solid Solution: \(Zn, Mg\) \(S, Se\)

The structural properties of the

PHONON SOFTENING AND SUPERCONDUCTIVITY IN TELLURIUM UNDER PRESSURE

Zn_xMg_{1-xS_ySe_{1-y}}