Analysis of a carbon dimer bound to a vacancy in iron using density functional theory and a new tight binding model

Recent density functional theory (DFT) calculations by Foerst et al. have predicted that vacancies in both low and high carbon steels have a carbon dimer bound to them. This is likely to change the thinking of metallurgists in the kinetics of the development of microstructures. While the notion of a C2 molecule bound to a vacancy in Fe will potentially assume a central importance in the atomistic modeling of steels, neither a recent tight binding (TB) model nor existing classical interatomic potentials can account for it. Here we present a new TB model for C in Fe, based on our earlier work for H in Fe, which correctly predicts the structure and energetics of the carbon dimer at a vacancy in Fe. Moreover the model is capable of dealing with both concentrated and dilute limits of carbon in both bcc-Fe and fcc-Fe as comparisons with DFT show. We use both DFT and TB to make a detailed analysis of the dimer and to come to an understanding as to what governs the choice of its curious orientation within the vacancy

Mean-Covariance Structure Models in Economic Research: an Application to a Lending Program for Development in Burkina Faso

As applied development economic models become more sophisticated, they include increasingly complex conceptual variables. Due to data collection limitations, accurate proxies and continuous variables are often unavailable. A Mean and Covariance Structure model (MECOSA) is offered as a useful methodology for the incorporation of latent variables with metric, censored metric, dichotomous and ordinal indicators. As an example, conceptual variables (including borrower homogeneity and the domino effect) presented in the Besley and Coate (1995) group lending repayment game were specified as latent variables with non-metric indicators. Data from 140 groups from a group lending program in Burkina Faso were used to demonstrate the application and interpretation of MECOSA.mean and covariance structure models, latent variables, development finance, group lending

Healthy Lifestyle Behaviors and Disparities between the United States mainland compared to Puerto Rico, Guam, and United States Virgin Islands (i. e., United States territories)

Background: No studies have compared the lifestyle behaviors between Puerto Rico, Guam, and the U. S. Virgin Islands to that of the United States mainland. Documenting and addressing health disparities between these geographically and culturally distinct areas are important public health objectives. Differences in health status between and among the United States mainland and territories merit systematic and careful analyses. Methods: Four key healthy lifestyle characteristics include tobacco use, body mass index, physical activity, and fruit/vegetable consumption. Data from the 2009 Behavioral Risk Factor Surveillance System (N=420,481) were used to examine United States mainland and territorial differences among the four key healthy lifestyle behaviors. Descriptive statistics were summarized with chi-square tests for independence and multiple adjusted logistic regression models were used to examine differences in health compliance rates while controlling for age, gender, income, and education. Frequencies determined whether Healthy People 2010 goals were met by each location. Results: Differences were found between the United States mainland and territories for smoking rates, body mass index, physical activity, and consumption of fruit/vegetables. None of the countries met all four Healthy People 2010 goals. Discussion: Even though, each location had unique challenges, Puerto Ricans’ health behaviors were significantly less favorable than residents in the other countries. We document prevalence rates and differences by country for each of the four healthy lifestyle characteristics. This study highlights the need for more research in these understudied areas as well as the importance of effective health promotion and disease prevention programs for all United States citizens including the mainland and all territories

Local Volume Effects in the Generalized Pseudopotential Theory

The generalized pseudopotential theory (GPT) is a powerful method for deriving real-space transferable interatomic potentials. Using a coarse-grained electronic structure, one can explicitly calculate the pair ion-ion and multi-ion interactions in simple and transition metals. Whilst successful in determining bulk properties, in central force metals the GPT fails to describe crystal defects for which there is a significant local volume change. A previous paper [PhysRevLett.66.3036 (1991)] found that by allowing the GPT total energy to depend upon some spatially-averaged local electron density, the energetics of vacancies and surfaces could be calculated within experimental ranges. In this paper, we develop the formalism further by explicitly calculating the forces and stress tensor associated with this total energy. We call this scheme the adaptive GPT (aGPT) and it is capable of both molecular dynamics and molecular statics. We apply the aGPT to vacancy formation and divacancy binding in hcp Mg and also calculate the local electron density corrections to the bulk elastic constants and phonon dispersion for which there is refinement over the baseline GPT treatment.Comment: 11 pages, 6 figure

H ingestion into He-burning convection zones in super-AGB stellar models as a potential site for intermediate neutron-density nucleosynthesis

We investigate the evolution of super-AGB (SAGB) thermal pulse (TP) stars for a range of metallicities (Z) and explore the effect of convective boundary mixing (CBM). With decreasing metallicity and evolution along the TP phase, the He-shell flash and the third dredge-up (TDU) occur closer together in time. After some time (depending upon the CBM parametrization), efficient TDU begins while the pulse-driven convection zone (PDCZ) is still present, causing a convective exchange of material between the PDCZ and the convective envelope. This results in the ingestion of protons into the convective He-burning pulse. Even small amounts of CBM encourage the interaction of the convection zones leading to transport of protons from the convective envelope into the He layer. H-burning luminosities exceed 10⁹ (in some cases 10¹⁰) L⊙. We also calculate models of dredge-out in the most massive SAGB stars and show that the dredge-out phenomenon is another likely site of convective-reactive H-¹²C combustion. We discuss the substantial uncertainties of stellar evolution models under these conditions. Nevertheless, the simulations suggest that in the convective-reactive H-combustion regime of H ingestion the star may encounter conditions for the intermediate neutron capture process (i-process). We speculate that some CEMP-s/r stars could originate in i-process conditions in the H ingestion phases of low-Z SAGB stars. This scenario would however suggest a very low electron-capture supernova rate from SAGB stars. We also simulate potential outbursts triggered by such H ingestion events, present their light curves and briefly discuss their transient properties

davos\texttt{davos}: a Python "smuggler" for constructing lightweight reproducible notebooks

Reproducibility is a core requirement of modern scientific research. For computational research, reproducibility means that code should produce the same results, even when run on different systems. A standard approach to ensuring reproducibility entails packaging a project's dependencies along with its primary code base. Existing solutions vary in how deeply these dependencies are specified, ranging from virtual environments, to containers, to virtual machines. Each of these existing solutions requires installing or setting up a system for running the desired code, increasing the complexity and time cost of sharing or engaging with reproducible science. Here, we propose a lighter-weight solution: the $\texttt{davos}$ package. When used in combination with a notebook-based Python project, $\texttt{davos}$ provides a mechanism for specifying (and automatically installing) the correct versions of the project's dependencies. The $\texttt{davos}$ package further ensures that those packages and specific versions are used every time the notebook's code is executed. This enables researchers to share a complete reproducible copy of their code within a single Jupyter notebook file

Changes in Producers’ Perceptions of Within-field Yield Variability Following Adoption of Cotton Yield Monitors

Precision Farming, Risk, Yield Monitor, Yield Variability, Yield Perceptions, Spatial Yield Distributions, Within Field Variability, Farm Management, Production Economics, Risk and Uncertainty, Q12, Q16,

Conservative Initial Mapping For Multidimensional Simulations of Stellar Explosions

Mapping one-dimensional stellar profiles onto multidimensional grids as initial conditions for hydrodynamics calculations can lead to numerical artifacts, one of the most severe of which is the violation of conservation laws for physical quantities such as energy and mass. Here we introduce a numerical scheme for mapping one-dimensional spherically-symmetric data onto multidimensional meshes so that these physical quantities are conserved. We validate our scheme by porting a realistic 1D Lagrangian stellar profile to the new multidimensional Eulerian hydro code CASTRO. Our results show that all important features in the profiles are reproduced on the new grid and that conservation laws are enforced at all resolutions after mapping.Comment: 7 pages, 5 figures, Proceeding for Conference on Computational Physics (CCP 2011