78 research outputs found
Software for quantum simulations of tomorrow
1noScientific software allows to transform theories and methods into numerical results and physical insight. This is especially true for the study of realistic models of materials, that heavily rely on first-principle computer simulations. This article describes the problems encountered in the development and maintenance of scientific software and examines the possible solutions. The ongoing work towards better software for present and future simulations of materials, performed with the Quantum ESPRESSO distribution in the framework of the EU-supported MaX Centre of Excellence, is introduced.openopenPaolo GiannozziGiannozzi, Paol
Thermal properties of materials from ab-initio quasi-harmonic phonons
This paper gives a short overview of the calculation of thermal properties of
materials from first principles, using the Quasi-Harmonic Approximation (QHA).
We first introduce some of the thermal properties of interest and describe how
they can be calculated in the framework of the QHA; then we briefly recall
Density-Functional Perturbation Theory as a tool for calculating phonons from
first principles, and present some codes that implement it; finally we review
recent applications of first-principle QHA.Comment: 23 pages, no figure
Carbonyl group generation on single-wall carbon nanotubes with nitric acid: A theoretical description
AbstractThe initial steps of single-wall carbon nanotube (SWNT) oxidation in nitric acid were studied using a (6,6) supercell with a mono-vacancy defect and employing spin-polarised density functional theory. According to our results, the geometric changes that occur during the process are significantly localised around the vacancy. The carbonyl group generation does not change the metallic nature of the nanosystem. Vibrational thermal corrections calculated using full and partial Hessian vibrational analysis indicated a small contribution to the reaction energy. An overall favourable oxidation pathway is proposed and includes an initial NO2+ exothermic electrophilic attack followed by an endothermic oxaziridine formation
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.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte
Position and momentum mapping of vibrations in graphene nanostructures in the electron microscope
Propagating atomic vibrational waves, phonons, rule important thermal,
mechanical, optoelectronic and transport characteristics of materials. Thus the
knowledge of phonon dispersion, namely the dependence of vibrational energy on
momentum is a key ingredient to understand and optimize the material's
behavior. However, despite its scientific importance in the last decade, the
phonon dispersion of a freestanding monolayer of two dimensional (2D) materials
such as graphene and its local variations has still remained elusive because of
experimental limitations of vibrational spectroscopy. Even though electron
energy loss spectroscopy (EELS) in transmission has recently been shown to
probe the local vibrational charge responses, these studies are yet limited to
polar materials like boron nitride or oxides, in which huge signals induced by
strong dipole moments are present. On the other hand, measurements on graphene
performed by inelastic x-ray (neutron) scattering spectroscopy or EELS in
reflection do not have any spatial resolution and require large microcrystals.
Here we provide a new pathway to determine the phonon dispersions down to the
scale of an individual freestanding graphene monolayer by mapping the distinct
vibration modes for a large momentum transfer. The measured scattering
intensities are accurately reproduced and interpreted with density functional
perturbation theory (DFPT). Additionally, a nanometre-scale mapping of selected
momentum (q) resolved vibration modes using graphene nanoribbon structures has
enabled us to spatially disentangle bulk, edge and surface vibrations
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.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic
Roadmap on Electronic Structure Codes in the Exascale Era
Electronic structure calculations have been instrumental in providing many
important insights into a range of physical and chemical properties of various
molecular and solid-state systems. Their importance to various fields,
including materials science, chemical sciences, computational chemistry and
device physics, is underscored by the large fraction of available public
supercomputing resources devoted to these calculations. As we enter the
exascale era, exciting new opportunities to increase simulation numbers, sizes,
and accuracies present themselves. In order to realize these promises, the
community of electronic structure software developers will however first have
to tackle a number of challenges pertaining to the efficient use of new
architectures that will rely heavily on massive parallelism and hardware
accelerators. This roadmap provides a broad overview of the state-of-the-art in
electronic structure calculations and of the various new directions being
pursued by the community. It covers 14 electronic structure codes, presenting
their current status, their development priorities over the next five years,
and their plans towards tackling the challenges and leveraging the
opportunities presented by the advent of exascale computing.Comment: Submitted as a roadmap article to Modelling and Simulation in
Materials Science and Engineering; Address any correspondence to Vikram
Gavini ([email protected]) and Danny Perez ([email protected]
Metallic, magnetic and molecular nanocontacts
Scanning tunnelling microscopy and break-junction experiments realize metallic and molecular nanocontacts that act as ideal one-dimensional channels between macroscopic electrodes. Emergent nanoscale phenomena typical of these systems encompass structural, mechanical, electronic, transport, and magnetic properties. This Review focuses on the theoretical explanation of some of these properties obtained with the help of first-principles methods. By tracing parallel theoretical and experimental developments from the discovery of nanowire formation and conductance quantization in gold nanowires to recent observations of emergent magnetism and Kondo correlations, we exemplify the main concepts and ingredients needed to bring together ab initio calculations and physical observations. It can be anticipated that diode, sensor, spin-valve and spin-filter functionalities relevant for spintronics and molecular electronics applications will benefit from the physical understanding thus obtained
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