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). The acronym 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 interoperable 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.

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

We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

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Scalable Molecular Dynamics with NAMD

We investigate Raman spectra of graphite oxide and functionalized graphene sheets with epoxy and hydroxyl groups and Stone−Wales and 5−8−5 defects by first-principles calculations to interpret our experimental results. Only the alternating pattern of single−double carbon bonds within the sp2 carbon ribbons provides a satisfactory explanation for the experimentally observed blue shift of the G band of the Raman spectra relative to graphite. To obtain these single−double bonds, it is necessary to have sp3 carbons on the edges of a zigzag carbon ribbon.

Raman Spectra of Graphite Oxide and Functionalized Graphene Sheets

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 pseudopotential 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 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

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Advanced capabilities for materials modelling with Quantum ESPRESSO.

Angle-resolved photoelectron spectroscopy and surface differential reflectivity have been used to study the electronic structure of Si(100):Sb-1\ifmmode\times\else\texttimes\fi{}1 and Si(100):Sb-2\ifmmode\times\else\texttimes\fi{}1 surfaces. For both surfaces, one occupied surface-state band has been mapped along the [010] and [011] directions. Both surfaces show a semiconducting behavior with a gap of 1.6 eV for the 1\ifmmode\times\else\texttimes\fi{}1-Sb and 1.4 eV for the 2\ifmmode\times\else\texttimes\fi{}1-Sb surface. A minimum-energy position at the Fermi level is derived for the empty surface-state band. The results are also compared with those obtained for the clean Si(100)2\ifmmode\times\else\texttimes\fi{}1 surface.

Si(100)1×1-Sb and Si(100)2×1-Sb surfaces studied with angle-resolved photoemission and surface differential reflectivity

Optical absorption of surface states at Si(111) 7 × 7

We perform density-functional calculations of the magnetic properties of a simplified structure aimed at capturing some of the features of the elusive $\ensuremath{\epsilon}$ phase of molecular oxygen at high pressure. Starting with the $\ensuremath{\delta}$ phase\char22{}which is a quasi-two-dimensional distorted triangular arrangement of antiferromagnetically ordered molecules\char22{}pressure could decrease the $b/a$ ratio in the basal planes pushing it toward the ideal triangular value of $1/\sqrt{3},$ thus increasing magnetic frustration. We conjecture that when frustration takes over, the magnetic order may turn into a $120\ifmmode^\circ\else\textdegree\fi{}$ planar spin-spiral structure inside the $\ensuremath{\epsilon}$ phase, until at higher pressures band-overlap metallization suppresses magnetization in the $\ensuremath{\zeta}$ phase. This conjecture is substantiated by calculations that also represent the attempt to apply state-of-the-art pseudopotential techniques to the magnetic properties of a frustrated antiferromagnet.

Noncolinear spin polarization from frustrated antiferromagnetism: A possible scenario for molecular oxygen at high pressure

By analyzing the distribution of revenues across the production sectors of quoted firms we suggest a novel dimension that drives the firms diversification process at country level. Data show a non trivial macro regional clustering of the diversification process, which underlines the relevance of geopolitical environments in determining the microscopic dynamics of economic entities. These findings demonstrate the possibility of singling out in complex ecosystems those micro-features that emerge at macro-levels, which could be of particular relevance for decision-makers in selecting the appropriate parameters to be acted upon in order to achieve desirable results. The understanding of this micro-macro information exchange is further deepened through the introduction of a simplified dynamic model.

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Guido L. Chiarotti

Papers

Si(100)1×1-Sb and Si(100)2×1-Sb surfaces studied with angle-resolved photoemission and surface differential reflectivity

Optical absorption of surface states at Si(111) 7 × 7

Noncolinear spin polarization from frustrated antiferromagnetism: A possible scenario for molecular oxygen at high pressure

Diversification versus Specialization in Complex Ecosystems

A parallel and modular deformable cell Car–Parrinello code