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

The Professional Learning Motivation Profile (PLMP): A Tool for Assessing Instructional Motivation

This article chronicles the collaboration of administrators from six districts and three college professors as they assessed professional learning during the first year of teaching. The examination led to the development of a Professional Learning Motivation Profile. Results from the profile indicated a traditional model of professional development was not effective in growing the professional learning motivation of beginning teachers. Anecdotal data shared includes how administrators used the data to inform conversations designed to support teachers in their journey toward courageous, effective instruction

A fully quantum mechanical calculation of the diffusivity of hydrogen in iron using the tight binding approximation and path integral theory

We present calculations of free energy barriers and diffusivities as functions of temperature for the diffusion of hydrogen in bcc-Fe. This is a fully quantum mechanical approach since the total energy landscape is computed using a new self consistent, transferable tight binding model for interstitial impurities in magnetic iron. Also the hydrogen nucleus is treated quantum mechanically and we compare here two approaches in the literature both based in the Feynman path integral formulation of statistical mechanics. We find that the quantum transition state theory which admits greater freedom for the proton to explore phase space gives result in better agreement with experiment than the alternative which is based on fixed centroid calculations of the free energy barrier. We also find results in better agreement compared to recent centroid molecular dynamics (CMD) calculations of the diffusivity which employed a classical interatomic potential rather than our quantum mechanical tight binding theory. In particular we find first that quantum effects persist to higher temperatures than previously thought, and conversely that the low temperature diffusivity is smaller than predicted in CMD calculations and larger than predicted by classical transition state theory. This will have impact on future modeling and simulation of hydrogen trapping and diffusion

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

Changing the First Lady’s Mystique: Defining the First Lady’s Legal Role and Upending Gender Norms

This Article explores the lack of formal guidelines governing the First Lady by first considering the history of the role and how the three branches of government have typically dealt with the role. Attention is also given to the possible intersection with the anti-nepotism statute when and if the First Lady acts as an advisor to the President. This Article then goes on to suggest that this lack of formality has allowed gender norms to govern the role. In an era where women’s rights have resurfaced as a central theme in political discourse, this Article concludes by suggesting some possible guidelines that may displace the gender norms that have been governing the role for far too long. Upending these gender norms from the White House would send a message that not only redefines the First Lady’s role but also redefines gender roles for the American public

Changing the First Lady\u27s Mystique: Defining the First Lady\u27s Legal Role and Upending Gender Norms

This Article explores the lack of formal guidelines governing the First Lady by first considering the history of the role and how the three branches of government have typically dealt with the role. Attention is also given to the possible intersection with the anti-nepotism statute when and if the First Lady acts as an advisor to the President. This Article then goes on to suggest that this lack of formality has allowed gender norms to govern the role. In an era where women’s rights have resurfaced as a central theme in political discourse, this Article concludes by suggesting some possible guidelines that may displace the gender norms that have been governing the role for far too long. Upending these gender norms from the White House would send a message that not only redefines the First Lady’s role but also redefines gender roles for the American public