arXiv subjects

S. de Gironcoli

Publications and source records attributed to S. de Gironcoli.

12 recordsLinked to original sources

arXiv2017

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.

cond-mat.mtrl-sci↗

arXiv2014

Projector augmented-wave and all-electron calculations across the periodic table: a comparison of structural and energetic properties

We construct a reference database of materials properties calculated using density-functional theory in the local or generalized-gradient approximation, and an all-electron or a projector augmented-wave (PAW) formulation, for verification and validation of first-principles simulations. All-electron calculations use the full-potential linearised augmented-plane wave method, as implemented in the \texttt{Elk} open-source code, while PAW calculations use the datasets developed by some of us in the open-source \texttt{PSlibrary} repository and the \texttt{Quantum ESPRESSO} distribution. We first calculate lattice parameters, bulk moduli, and energy differences for alkaline metals, alkaline earths, and $3d$ and $4d$ transition metals in three ideal, reference phases (simple cubic, fcc, and bcc), representing a standardized crystalline monoatomic solid-state test. Then, as suggested by K. Lejaeghere {\it et al.}, [Critical Reviews in Solid State and Material Sciences 39, p 1 (2014)], we compare the equations of state for all elements, except lanthanides and actinides, in their experimental phase (or occasionally a simpler, closely related one). PAW and all-electron energy differences and structural parameters agree in most cases within a few meV/atom and a fraction of a percent, respectively. This level of agreement, comparable with the previous study, includes also other PAW and all-electron data from the electronic-structure codes \texttt{VASP} and \texttt{WIEN2K}, and underscores the overall reliability of current, state-of-the-art electronic-structure calculations. At the same time, discrepancies that arise even within the same formulation for simple, fundamental structural properties point to the urgent need of establishing standards for verification and validation, reference data sets, and careful refinements of the computational approaches used.

cond-mat.mtrl-sci↗

arXiv2009

Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials

Quantum ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn Source Package for Research in Electronic Structure, Simulation, and Optimization". It is freely available to researchers around the world under the terms of the GNU General Public License. Quantum ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively-parallel architectures, and a great effort being devoted to user friendliness. Quantum ESPRESSO is evolving towards a distribution of independent and inter-operable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

cond-mat.mtrl-sci↗

arXiv2004

Structure and dynamics of oxygen adsorbed on Ag(100) vicinal surfaces

The structure and dynamics of atomic oxygen adsorbed on Ag(410) and Ag(210) surfaces have been investigated using density functional theory. Our results show that the adsorption configuration in which O adatoms decorate the upper side of the (110) steps forming O--Ag--O rows is particularly stable for both surfaces. On Ag(210), this arrangement is more stable than other configurations at all the investigated coverages. On Ag(410), adsorption on the terrace and at the step edge are almost degenerate, the former being slightly preferred at low coverage while the latter is stabilized by increasing the coverage. These findings are substantiated by a comparison between the vibrational modes, calculated within density-functional perturbation theory, and the HREEL spectrum which has been recently measured in these systems.

cond-mat.mtrl-sci↗

arXiv2003

On-surface and Subsurface Adsorption of Oxygen on Stepped Ag(210) and Ag(410) Surfaces

The adsorption of atomic oxygen and its inclusion into subsurface sites on Ag(210) and Ag(410) surfaces have been investigated using density functional theory. We find that--in the absence of adatoms on the first metal layer--subsurface adsorption results in strong lattice distortion which makes it energetically unfavoured. However subsurface sites are significantly stabilised when a sufficient amount of O adatoms is present on the surface. At high enough O coverage on the Ag(210) surface the mixed on-surface + subsurface O adsorption is energetically favoured with respect to the on-surface only adsorption. Instead, on the Ag(410) surface, at the coverage we have considered (3/8 ML), the existence of stable terrace sites makes the subsurface O incorporation less favourable. These findings are compatible with the results of recent HREEL experiments which have actually motivated this work.

cond-mat↗

arXiv2003

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 part of the e-p self energy are calculated for the surface state in the binding energy range from the $\barΓ$ 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 real part of the self-energy presents a strong temperature and energy variation close to $E_{F}$, making it difficult to measure its value just at $E_{F}$.

cond-mat.mtrl-sci↗

arXiv2002

Disproportionation Phenomena on Free and Strained Sn/Ge(111) and Sn/Si(111) Surfaces

Distortions of the $\sqrt3\times\sqrt3$ Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, $Q$, related to the surface band occupancy. A novel understanding of the $(3\times3)$-1U (``1 up, 2 down'') and 2U (``2 up, 1 down'') distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distorsions. Negative strain attracts pseudocharge from the valence band causing first a $(3\times3)$-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a $(\sqrt3\times\sqrt3)$-3U (``all up'') state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.

cond-mat.mtrl-sci↗

arXiv2000

Phonons and related properties of extended systems from density-functional perturbation theory

This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudo-potential 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 wave-length 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.

cond-mat.mtrl-sci↗

arXiv1999

The mechanism for the 3 x 3 distortion of Sn/ge (111)

We show that two distinct $3 \times 3$ ground states, one nonmagnetic, metallic, and distorted, the other magnetic, semimetallic (or insulating) and undistorted, compete in $α$-phase adsorbates on semiconductor (111) surfaces. In Sn/Ge(111), LSDA/GGA calculations indicate, in agreement with experiment, that the distorted metallic ground state prevails. The reason for stability of this state is analysed, and is 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.

cond-mat↗

arXiv1999

Floating bonds and gap states in a-Si and a-Si:H from first principles calculations

We study in detail by means of ab-initio pseudopotential calculations the electronic structure of five-fold coordinated (T_5) defects in a-Si and a-Si:H, also during their formation and their evolution upon hydrogenation. The atom-projected densities of states (DOS) and an accurate analysis of the valence charge distribution clearly indicate the fundamental contribution of T_5 defects in originating gap states through their nearest neighbors. The interaction with hydrogen can reduce the DOS in the gap annihilating T_5 defects.

cond-mat.mtrl-sci↗

arXiv1999

Coordination defects in a-Si and a-Si:H : a characterization from first principles calculations

We study by means of first-principles pseudopotential method the coordination defects in a-Si and a-Si:H, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the ``electron localization function'' (ELF) allows to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly three-fold (T_3) and five-fold (T_5) coordinated defects. We found that electronic states in the gap can be associated to both kind of defects, and that in both cases the interaction with hydrogen can reduce the density of states in the gap.

cond-mat.mtrl-sci↗

arXiv1996

Phonon Softening and Elastic Instabilities in the Cubic-to-Orthorhombic Structural Transition of CsH

The cubic-to-orthorhombic structural transition occurring in CsH at a pressure of about 17 GPa is studied by ab initio calculations. The relative stability of the competing structures and the transition pressure are correctly predicted. We show that this pressure-induced first-order transition is intimately related to a displacive second-order transition which would occur upon application of a shear strain to the (110) planes. The resulting instability is rationalized in terms of the pressure-induced modifications of the electronic structure.

cond-mat.mtrl-sci↗