4,996 research outputs found
Preparation of monotectic alloys having a controlled microstructure by directional solidification under dopant-induced interface breakdown
Monotectic alloys having aligned spherical particles of rods of the minor component dispersed in a matrix of the major component are prepared by forming a melt containing predetermined amounts of the major and minor components of a chosen monotectic system, providing in the melt a dopant capable of breaking down the liquid solid interface for the chosen alloy, and directionally solidfying the melt at a selected temperature gradient and a selected rate of movement of the liquid-solid interface (growth rate). Shaping of the minor component into spheres or rods and the spacing between them are controlled by the amount of dopant and the temperature gradient and growth rate values. Specific alloy systems include Al Bi, Al Pb and Zn Bi, using a transition element such as iron
SOYBEAN TRADER: A MICROCOMPUTER SIMULATION OF INTERNATIONAL AGRICULTURAL TRADE
Soybean Trader is a microcomputer simulation of international grain trading. The program uses the format of a graphics-oriented game to teach basic economic principles and to stimulate interest in agricultural trade. Profits from trading serve as a score, and competition is encouraged by ranking top scores in Trader's Hall of Fame. Results of tests with adult and youth audiences indicated that the program is an interesting and effective teaching tool.International Relations/Trade,
The Impacts of Foot and Mouth Disease Outbreaks on the Brazilian Meat Market
This study uses unrestricted vector autoregression method and historical decomposition with directed acyclic graphs to quantity the impacts of the foot and mouth disease outbreak on the Brazilian meat market for different levels of the industry (export, wholesale and farm). The imposition of an import ban by Russia on Brazilian meat exports is also analyzed. Results show that beef, pork, and chicken export prices all decreased after the FMD outbreak. More importantly, our findings indicate that all prices ended up recovering after the removal of the import ban by Russia in December 2007. As for the price margins in both beef and pork meat supply chains, the export-to-farm and export-to-wholesale margins were found to be very close to each other. On the other hand, the chicken price margin at the export level relative to the farm and wholesale levels had opposing directions trend movements. Finally, the historical decomposition of analysis of the export beef price revealed that the removal of the Russian ban on Brazilian meat imports had a very drastic positive influence on the beef industry supply chain in general.foot and mouth disease, Brazilian meat market, vector autoregression model, historical decomposition, directed acyclic graphs., Food Consumption/Nutrition/Food Safety, International Relations/Trade, Livestock Production/Industries,
A systematic benchmark of the ab initio Bethe-Salpeter equation approach for low-lying optical excitations of small organic molecules
The predictive power of the ab initio Bethe-Salpeter equation (BSE) approach,
rigorously based on many-body Green's function theory but incorporating
information from density functional theory, has already been demonstrated for
the optical gaps and spectra of solid-state systems. Interest in photoactive
hybrid organic/inorganic systems has recently increased, and so has the use of
the BSE for computing neutral excitations of organic molecules. However, no
systematic benchmarks of the BSE for neutral electronic excitations of organic
molecules exist. Here, we study the performance of the BSE for the 28 small
molecules in Thiel's widely-used time-dependent density functional theory
benchmark set [M. Schreiber et al. J. Chem. Phys. 128, 134110 (2008)]. We
observe that the BSE produces results that depend critically on the mean-field
starting point employed in the perturbative approach. We find that this
starting point dependence is mainly introduced through the quasiparticle
energies obtained at the intermediate GW step, and that with a judicious choice
of starting mean-field, singlet excitation energies obtained from BSE are in
excellent quantitative agreement with higher-level wavefunction methods. The
quality of the triplet excitations is slightly less satisfactory
Optimized Effective Potentials in Finite Basis Sets
The finite basis optimized effective potential (OEP) method within density
functional theory is examined as an ill-posed problem. It is shown that the
generation of nonphysical potentials is a controllable manifestation of the use
of unbalanced, and thus unsuitable, basis sets. A modified functional
incorporating a regularizing smoothness measure of the OEP is introduced. This
provides a condition on balanced basis sets for the potential, as well as a
method to determine the most appropriate OEP potential and energy from
calculations performed with any finite basis set.Comment: 23 pages, 28 figure
Classical dynamics of triatomic systems: Energized harmonic molecules
The dynamical assumptions underlying the Slater and RRK classical-mechanical theories of unimolecular reaction rates are investigated. The predictions of these theories for several nonlinear, triatomic, harmonically bonded molecular models are compared with the results obtained from the integration of the classical equations of motion. The accuracy of the small-vibration and weak-coupling assumptions are found to break down at energies above about one-quarter of a bond dissociation energy. Nonetheless, the small-vibration approximation predicts reaction frequencies in good agreement with the exact results for the models. The effects of rotation on intramolecular energy exchange are examined and found to be significant
Density-density functionals and effective potentials in many-body electronic structure calculations
We demonstrate the existence of different density-density functionals
designed to retain selected properties of the many-body ground state in a
non-interacting solution starting from the standard density functional theory
ground state. We focus on diffusion quantum Monte Carlo applications that
require trial wave functions with optimal Fermion nodes. The theory is
extensible and can be used to understand current practices in several
electronic structure methods within a generalized density functional framework.
The theory justifies and stimulates the search of optimal empirical density
functionals and effective potentials for accurate calculations of the
properties of real materials, but also cautions on the limits of their
applicability. The concepts are tested and validated with a near-analytic
model.Comment: five figure
Novel properties of the Kohn-Sham exchange potential for open systems: application to the two-dimensional electron gas
The properties of the Kohn-Sham (KS) exchange potential for open systems in
thermodynamical equilibrium, where the number of particles is non-conserved,
are analyzed with the Optimized Effective Potential (OEP) method of Density
Functional Theory (DFT) at zero temperature. The quasi two-dimensional electron
gas (2DEG) is used as an illustrative example. The main findings are that the
KS exchange potential builds a significant barrier-like structure under slight
population of the second subband, and that both the asymptotic value of the KS
exchange potential and the inter-subband energy jump discontinuously at the
one-subband (1S) -> two-subband (2S) transition. The results obtained in this
system offer new insights on open problems of semiconductors, such as the
band-gap underestimation and the band-gap renormalization by photo-excited
carriers.Comment: 7 pages, 3 figures, uses epl.cls(included), accepted for publication
in Europhysics Letter
Ground state of two electrons on concentric spheres
We extend our analysis of two electrons on a sphere [Phys. Rev. A {\bf 79},
062517 (2009); Phys. Rev. Lett. {\bf 103}, 123008 (2009)] to electrons on
concentric spheres with different radii. The strengths and weaknesses of
several electronic structure models are analyzed, ranging from the mean-field
approximation (restricted and unrestricted Hartree-Fock solutions) to
configuration interaction expansion, leading to near-exact wave functions and
energies. The M{\o}ller-Plesset energy corrections (up to third-order) and the
asymptotic expansion for the large-spheres regime are also considered. We also
study the position intracules derived from approximate and exact wave
functions. We find evidence for the existence of a long-range Coulomb hole in
the large-spheres regime, and infer that unrestricted Hartree-Fock theory
over-localizes the electrons.Comment: 10 pages, 10 figure
Managing Risk in a Dynamic World Economy
4 pp.,1 tableThis publication provides an overview of the globalization of U.S. agriculture, with special emphasis on implications for risk management. To be successful in a rapidly changing global environment, farmers will need a clear understanding of risk and how to manage it
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