44 research outputs found

    Vibrational anharmonicity of small gold and silver clusters using the VSCF method

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    We study the vibrational spectra of small neutral gold (Au2–Au10) and silver (Ag2–Au5) clusters using the vibrational self-consistent field method (VSCF) in order to account for anharmonicity. We report harmonic, VSCF, and correlation-corrected VSCF calculations obtained using a vibrational configuration interaction approach (VSCF/VCI). Our implementation of the method is based on an efficient calculation of the potential energy surfaces (PES), using periodic density functional theory (DFT) with a plane-wave pseudopotential basis. In some cases, we use an efficient technique (fast-VSCF) assisted by the Voter–Chen potential in order to get an efficient reduction of the number of pair-couplings between modes. This allows us to efficiently reduce the computing time of 2D-PES without degrading the accuracy. We found that anharmonicity of the gold clusters is very small with maximum rms deviations of about 1 cm−1, although for some particular modes anharmonicity reaches values slightly larger than 2 cm−1. Silver clusters show slightly larger anharmonicity. In both cases, large differences between calculated and experimental vibrational frequencies (when available) stem more likely from the quality of the electronic structure method used than from vibrational anharmonicity. We show that noble gas embedding often affects the vibrational properties of these clusters more than anharmonicity, and discuss our results in the context of experimental studies

    The nature and role of the gold-krypton interactions in small neutral gold clusters

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    © 2015 American Chemical Society. We investigate the nature and role of krypton embedding in small neutral gold clusters. For some of these clusters, we observe a particular site-dependent character of the Kr binding that does not completely follow the criterion of binding at low-coordinated sites, widely accepted for interaction of a noble gas with closed-shell metal systems such as metal surfaces. We aim at understanding the effect of low dimensionality and open-shell electronic structure of the odd-numbered clusters on the noble gas-metal cluster interaction. First, we investigate the role of attractive and repulsive forces, and the frontier molecular orbitals. Second, we investigate the Au-Kr interaction in terms of reactivity and bonding character. We use a reactivity index derived from Fukui formalism, and criteria provided by the electron localization function (ELF), in order to classify the type of bonding. We carry out this study on the minimum energy structures of neutral gold clusters, as obtained using pseudo potential plane-wave density functional theory (DFT). A model is proposed that includes the effect of attractive electrostatic, van der Waals and repulsive forces, together with effects originating from orbital overlap. This satisfactorily explains minimum configurations of the noble gas-gold cluster systems, the site preference of the noble gas atoms, and changes in electronic properties

    An alternative methodology to assess the quality of empirical potentials for small gold clusters

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    We present a methodology based on local comparisons of potential energy surfaces (PES) in order to assess the quality of empirical potentials. We compare five typical empirical potentials using a criterion that shows which of these potentials resembles better a PES obtained with a high-level electronic structure method. The methodology relies on a many-body expansion in terms of normal coordinates of both the empirical and high-level theory PES. Then we investigate in a systematical way, how the features of the reference high-level theory PES are reproduced by each empirical potential in the vicinity of a given minimum energy structure. We use plane-wave density functional theory (DFT) as a reference, in particular the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional and an ultrasoft Vanderbilt pseudo potential. This study is carried out on neutral gold clusters with up to five atoms

    Image Restoration Using Space-Variant Gaussian Scale Mixtures in Overcomplete Pyramids

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    Design and construction of tank-chassis and lifting structure for centrifugal pump HL260 M powered by a Diesel Engine

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    This article deals with the design, simulation and construction of a fuel storage tank-chassis and a lifting system coupled as a single unit to a Cummins QSK19 engine driven HL260m pump that can guarantee an operating autonomy of up to 12 continuous hours and can be transported to different locations by means of lifting systems. For the mechanical design the recommendations of the American Institute of Steel Construction (AISC) and the application of the failure criteria for Von Mises ductile materials or Maximum Energy Distortion were used. For the dimensioning of the storage tank, the average consumption stipulated by the manufacturer was used and the simulations were performed with SolidWorks®. A functional and safe system that can be used in on-site applications was achieved

    Biosynthesis of Silver Nanoparticles Using Chenopodium ambrosioides

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    Biosynthesis of silver nanoparticles (AgNPs) was achieved using extract of Chenopodium ambrosioides as a reducer and coating agent at room temperature (25°C). Two molar solutions of AgNO3 (1 mM and 10 mM) and five extract volumes (0.5, 1, 2, 3, and 5 mL) were used to assess quantity, shape, and size of the particles. The UV-Vis spectra gave surface plasmon resonance at 434–436 nm of the NPs synthesized with AgNO3 10 mM and all extract volumes tested, showing a direct relationship between extract volumes and quantity of particles formed. In contrast, the concentration of silver ions was related negatively to particle size. The smallest (4.9 ± 3.4 nm) particles were obtained with 1 mL of extract in AgNO3 10 mM and the larger amount of particles were obtained with 2 mL and 5 mL of extract. TEM study indicated that the particles were polycrystalline and randomly oriented with a silver structure face centered cubic (fcc) and fourier transform infrared spectroscopy (FTIR) indicated that disappearance of the –OH group band after bioreduction evidences its role in reducing silver ions

    Study of the Performance of the Organic Extracts of Chenopodium ambrosioides

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    There are many ways to obtain metal nanoparticles: biological, physical, and chemical ways and combinations of these approaches. Synthesis assisted with plant extracts has been widely documented. However, one issue that is under discussion refers to the metabolites responsible for reduction and stabilization that confine nanoparticle growth and prevent coalescence between nanoparticles in order to avoid agglomeration/precipitation. In this study, Ag nanoparticles were synthesized using organic extracts of Chenopodium ambrosioides with different polarities (hexane, dichloromethane, and methanol). Each extract was phytochemically characterized to identify the nature of the metabolites responsible for nanoparticle formation. With methanol extract, the compounds responsible for reducing and stabilizing silver nanoparticle were associated with the presence of phenolic compounds (flavonoids and tannins), while, with dichloromethane and hexane extracts, the responsible compounds were mainly terpenoids. Large part of the reducing activity of secondary metabolites in C. ambrosioides is closely related to compounds with antioxidant capacity, such as phenolic compounds (flavone glycoside and isorhamnetin), which are the main constituents of the methanol extracts. Otherwise, terpenoids (trans-diol, α-terpineol, monoterpene hydroperoxides, and apiole) are the central metabolites present in dichloromethane and hexane extracts

    Lecturas de política monetaria y financiera

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    1 archivo PDF (487 páginas)Conjunto de trabajos de investigadores especializados en teoría monetaria y financiera, que presentan un análisis teórico y empírico de los cambios de la economía mexicana, propuestas y críticas tanto de los procesos mismos cómo de las políticas con que enfrentan las autoridades monetarias nacionales el reto que implica la mayor integración del sistema mexicano al internacional. Las investigaciones se agrupan en seis secciones temáticas: Política monetaria y cambiaria; Influencia de los flujos internacionales de capital; La política monetaria y sus relaciones con la inflación; Sector financiero, crisis y política económica; Sector bursátil y Marco Legal
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