Study of the triplet and pair structure of strong electrolytes modeled via truncated Coulomb interactions

9 pages, 10 figures, 1 table.The structure of 3:3 and 1:3 electrolyte solutions at various concentrations and several cation/anion size ratios has been analyzed in terms of triplet and pair correlation functions, by means of simulation and a triplet integral equation theory derived from the inhomogeneous Ornstein–Zernike equation. The interaction model consists of a truncated and shifted Coulomb plus the Ramanathan–Friedman repulsive core. Concentration and size and charge asymmetry are found to induce changes in the triplet structure beyond those predicted by the simple superposition approximation, which are, however, correctly reproduced by the triplet integral equation.This work was funded by the Spanish Dirección General de Enseñanza Superior e Investigación Científica under Grant No. BFM2001-1017-C03. S.J. acknowledges financial support from the Universidad Complutense de Madrid.Peer reviewe

Theory and simulation of central force model potentials: Application to homonuclear diatomic molecules

14 pags., 14 figs., 6 tabs.Structure and thermodynamics of fluids made of particles that interact via a central force model potential are studied by means of Monte Carlo simulations and integral equation theories. The Hamiltonian has two terms, an intramolecular component represented by a harmonic oscillatorlike potential and an intermolecular interaction of the Lennard-Jones type. The potential does not fulfill the steric saturation condition so it leads to a polydisperse system. First, we investigate the association (clustering) and thermodynamic properties as a function of the potential parameters, such as the intramolecular potential depth, force constant, and bond length. It is shown that the atomic hypernetted chain (HNC) integral equation provides a correct description of the model as compared with simulation results. The calculation of the HNC pseudospinodal curve indicates that the stability boundaries between the vapor and liquid phases are strongly dependent on the bond length and suggests that there might be a direct gas-solid transition for certain elongations. On the other hand, we have assessed the ability of the model to describe the thermodynamics and structure of diatomic liquids such as N2 and halogens. To this end we have devised a procedure to model the intramolecular potential depth to reproduce the complete association limit (i.e., an average number of bonds per particle equal to one). This constraint is imposed on the Ornstein-Zernike integral equation in a straightforward numerical way. The structure of the resulting fluid is compared with results from molecular theories. An excellent agreement between the HNC results for the associating fluid and the reference interaction site model (RISM)-HNC computations for the atom-atom model of the same fluid is obtained. There is also a remarkable coincidence between the simulation results for the molecular and the associating liquids, despite the polydisperse character of the latter. The stability boundaries in the complete association limit as predicted by the HNC integral equation have been computed for different bond lengths corresponding to real molecular liquids. These boundaries appear close to the experimental liquid branch of the vapor-liquid coexistence line of the molecular systems under consideration. © 1996 American Institute of Physics.This work was partially supported by Grants No. PB93- 0085 and PB94-0112 furnished by the Direccion General de Investigacion Cientıfica y Tecnologica ~DGICYT! of Spain. FB acknowledges a predoctoral fellowship awarded by the Universidad Complutense de Madrid

An integral equation approach to orientational phase transitions in two and three dimensional disordered systems

The use of inhomogeneous Ornstein-Zernike equations to analyze phase transitions and ordered phases in magnetic systems is explored both in bulk three dimensional disordered Heisenberg systems and in a simple model for a two dimensional ferrofluid monolayer. In addition to closures like the Mean Spherical Approximation, Hypernetted Chain and Zerah-Hansen approximation, the inhomogeneous Ornstein-Zernike equation must be complemented by a one-body closure, for which the Born-Green equation has been used in this paper. The results obtained prove that the proposed approach can furnish accurate estimates for the paramagneticferromagnetic transition in the three dimensional Heisenberg spin fluid, reproducing reliably the structure of the isotropic and ordered phases. In two dimensions, the results are fairly accurate as well, both for the dipolar film alone and in the presence of external perpendicular fields. At high densities/dipole moments the equation seems to predict a transition to a phase in which the dipoles lie mostly in the plane and are aligned into vortex-like structures. Evidence of this new phase is found in the simulation at somewhat higher couplingsВикористання неоднорідних рівнянь Орнштейна-Церніке для вивчення фазових переходів і впорядкованих фаз в магнітних системах досліджується як у невпорядкованих гайзенбергівських системах так і в простій моделі для двовимірного ферофлюїдного моношару. Неоднорідне рівняння Орнштейна-Церніке, крім таких замикань як середньосферичне, гіперланцюгове і наближення Зера-Гансена, мусить бути доповнене одно-частинковим замиканням, для якого було використано в цій статті рівняння Борна-Гріна. Отримані результати доводять, що запропонований підхід може давати точні оцінки для переходу парамагнетик-феромагнетик в тривимірному гайзенбергівському спіновому флюїді, надійно відтворюючи структуру ізотропної і впорядкованої фаз. У двох вимірах, результати є, безумовно, точними як для дипольної плівки без поля, так і в присутності зовнішніх перпендикулярно направлених полів. При високих густинах/дипольних моментах рівняння передбачають перехід до фази, в якій диполі лежать в основному в площині і утворюють вихороподібні структури. Наявність цієї нової фази є знайдена при дещо сильніших параметрах при моделюванні

Phase diagrams of classical spin fluids: the influence of an external magnetic field on the liquid-gas transition

The influence of an external magnetic field on the liquid-gas phase transition in Ising, XY, and Heisenberg spin fluid models is studied using a modified mean field theory and Gibbs ensemble Monte Carlo simulations. It is demonstrated that the theory is able to reproduce quantitatively all characteristic features of the field dependence of the critical temperature T_c(H) for all the three models. These features include a monotonic decrease of T_c with rising H in the case of the Ising fluid as well as a more complicated nonmonotonic behavior for the XY and Heisenberg models. The nonmonotonicity consists in a decrease of T_c with increasing H at weak external fields, an increase of T_c with rising H in the strong field regime, and the existence of a minimum in T_c(H) at intermediate values of H. Analytical expressions for T_c(H) in the large field limit are presented as well. The magnetic para-ferro phase transition is also considered in simulations and described within the mean field theory.Comment: 14 pages, 12 figures (to be submitted to Phys. Rev. E

A farming systems approach to linking agricultural policies with biodiversity and ecosystem services

Concepts and QuestionsMany countries are reshaping their agricultural policies to better enhance biodiversity and ecosystem services (BES) in farmlands, but measuring the effectiveness of policy instruments in BES delivery is challenging. Using the European Agricultural Policy as an example, we propose the application of a farming systems (FS) approach as a cost-effective tool for linking policy design and expected BES outcomes. On the basis of available data from subsidy payment agencies, such an approach can identify groups of farms that share similar management practices as well as the associations between FS and corresponding BES potential, and improve modeled outputs of farm management responses to policies and other drivers of change. We describe how this relatively unexplored source of information can help to support applied ecological research and relevant policy, and call for these data to be made available across Europe and elsewhereN/

Impacts of sheep versus cattle livestock systems on birds of Mediterranean grasslands

Mediterranean pastures are experiencing strong changes in management, involving shifts from sheep to cattle-based livestock systems. The impacts of such shifts on biodiversity are still poorly understood. Here, we sought to contrast the grazing regime, vegetation structure, bird species richness and abundance, between sheep and cattle grazed parcels, to understand the mechanisms through which management decisions impact farmland birds. During spring 2019, we characterized livestock management, bird populations and sward structure in 23 cattle and 27 sheep grazed parcels. We used a Structural Equation Model to infer the direct and indirect effects of sheep and cattle grazing on birds. Although no effects were found on overall species richness, there were species-specific responses to sheep and cattle grazed systems. Grazing pressure (variable integrating stocking rate and the number of days in the parcel) had negative impacts on the prevalence/abundance of Zitting Cisticola, Corn Bunting and Little Bustard, either directly or indirectly, through the effects of grazing pressure on vegetation height. Animal density and vegetation cover had direct positive effects in Galerida spp. and Common Quail, respectively. Zitting Cisticola and Little Bustard also showed a direct response to livestock type. Our study emphasizes the importance of grazing pressure as a driver of negative impacts for bird populations in Mediterranean grasslands. Since the ongoing transition from sheep to cattle-based systems involves increases in stocking rate, and therefore potentially higher grazing pressure, we propose a policy change to cap the maximum allowed grazing pressure. At the landscape scale, a mix of sheep and cattle grazed fields would be beneficial for maintaining bird diversityinfo:eu-repo/semantics/publishedVersio

On the triplet structure of binary liquids

An approach to calculate the triplet structure of a simple liquid, that was proposed some years ago by Barrat, Hansen, and Pastore ͓Phys. Rev. Lett. 58, 2075 ͑1987͔͒ has been tested in the binary case. This approach is based on a factorization ansatz for the triplet direct correlation function c (3) ; the unknown factor function is determined via the sum rule relating c (3) and the pair direct correlation function which is the only input information of the system that is required in this formalism. We present an efficient and stable numerical algorithm which solves the six ͑partly coupled͒ integral equations for the unknown factor functions. Results are given for the case of a binary hard-sphere mixture and complemented by computer simulation data

On the triplet structure of binary liquids

An approach to calculate the triplet structure of a simple liquid, that was proposed some years ago by Barrat, Hansen, and Pastore ͓Phys. Rev. Lett. 58, 2075 ͑1987͔͒ has been tested in the binary case. This approach is based on a factorization ansatz for the triplet direct correlation function c (3) ; the unknown factor function is determined via the sum rule relating c (3) and the pair direct correlation function which is the only input information of the system that is required in this formalism. We present an efficient and stable numerical algorithm which solves the six ͑partly coupled͒ integral equations for the unknown factor functions. Results are given for the case of a binary hard-sphere mixture and complemented by computer simulation data

The communication of educational research. An approach to the relationship between research, dissemination and teacher practice

Este artículo presenta una investigación que busca conocer las visiones que docentes no universitarios tienen de dos tipos de artículos de investigación educativa: uno más académico y otro de difusión, y, de esta forma, determinar si los artículos de divulgación están contribuyendo a acercar la investigación a los profesores. Para ello se replican y adaptan los estudios de Zeuli (1994) y Bartels (2003). Concretamente, se realizó un estudio cualitativo en el que se les pidió a veinte docentes experimentados (diez de Primaria, diez de Secundaria; la mitad iniciados en investigación, la mitad no) que leyeran dos artículos de investigación uno de difusión y otro más académico pero de contenido similar. Posteriormente se les hizo una entrevista individual semiestructurada para conocer sus opiniones. Los resultados, en coherencia con otros estudios, apuntan a que los docentes son críticos con los artículos ya que valoran negativamente la falta de utilidad de los resultados, critican el lenguaje utilizado, consideran que los estudios son de escaso interés y de baja calidad. Además, los artículos de difusión, en general, no logran disminuir la distancia entre la investigación y la práctica educativa.This article presents an investigation that seeks to identify the views that no university teachers have two types of educational research articles: a more academic and other of disseminating, and, thus, determine whether the dissemination articles are helping to bring research to teachers. To achieve this were replicate and adapt the studies of Zeuli (1994) and Bartels (2003). Specifically, was made a qualitative study in which they were asked to twenty experienced teachers (ten Primary ten secondary, half-initiated research, half not) to read two research papers one of dissemination and other more academic but similar theme. After that it was conducted an interview with each of them. The results, consistent with other studies, suggest that teachers are critical articles and negatively valued the lack of usefulness of the results, criticize the language used, they consider that studies are of little interest and low quality. Also, the dissemination articles generally fail to bridge the gap between research and educational practice.Universidad de Granada. Departamento de Didáctica y Organización Escolar. Grupo FORCE (HUM386