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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.
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Metallo-organic molecules with highly conjugated pi-electrons, like phthalocyanines (Pc's), are widely investigated for usage in electronic and electro-optic devices. However, their weak coupling with semiconductors is an obstacle to technological applications. Here we report a first-principle theoretical study of some fundamental features of the Pc-semiconductor interaction. Our results shed light on the general problem of organic-inorganic coupling and show that an effective coupling can be achieved by a careful choice of the Pc-substrate system and the semiconductor doping. Our results also reveal a universal alignment of the Pc electronic levels to the semiconductor band gap and suggest a general procedure for designing efficiently coupled organic-inorganic systems.
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Indóis/química , Metaloporfirinas/química , Semicondutores , Isoindóis , Modelos Moleculares , Conformação Molecular , Titânio/químicaRESUMO
We present a plane-wave ultrasoft pseudopotential implementation of first-principle molecular dynamics, which is well suited to model large molecular systems containing transition metal centers. We describe an efficient strategy for parallelization that includes special features to deal with the augmented charge in the contest of Vanderbilt's ultrasoft pseudopotentials. We also discuss a simple approach to model molecular systems with a net charge and/or large dipole/quadrupole moments. We present test applications to manganese and iron porphyrins representative of a large class of biologically relevant metalorganic systems. Our results show that accurate density-functional theory calculations on systems with several hundred atoms are feasible with access to moderate computational resources.
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Biologia Computacional , Metaloporfirinas/química , Sítios de Ligação , Ferro/química , Manganês/química , Conformação Molecular , Mioglobina/química , Mioglobina/metabolismo , TermodinâmicaRESUMO
In GaAsyN(1-y), the presence of a few percent of N induces a large reduction of the GaAs band gap that vanishes upon hydrogenation. In the present Letter, the energetics of N-H complexes and their effects on the band structure of the GaAs0.97N0.03 alloy have been investigated by first-principles density functional methods. We find that monohydrogen N-H+ and dihydrogen N-H* 2 complexes are formed depending on doping. Moreover, only N-H* 2 complexes account for the neutralization of nitrogen effects. A model is proposed that clarifies the passivation mechanism of nitrogen by H.
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The effects of external and internal strains and of defect charges on the formation of gallium vacancies and arsenic antisites in GaAs and In0.5Ga0.5As have been investigated by ab initio density functional methods. Present results show that a proper understanding of strain and defect charge permits the development of a defect engineering of semiconductors. Specifically, they predict that arsenic antisites in InGaAs ternary alloys can form, upon p-type doping in the presence of an arsenic overpressure, even in the case of high-temperature epitaxial growths.
