Density, structure and dynamics of water: the effect of Van der Waals interactions

It is known that ab initio molecular dynamics (AIMD) simulations of liquid water, based on the generalized gradient approximation (GGA) to density functional theory (DFT), yield structural and diffusive properties in reasonable agreement with experiment only if artificially high temperatures are used in the simulations. The equilibrium density, at normal conditions, of DFT water has been recently shown by Schmidt et al. [J. Phys. chem. B, 113, 11959 (2009)] to be underestimated by different GGA functionals for exchange and correlation, and corrected by the addition of interatomic pair potentials to describe van derWaals (vdW) interactions. In this contribution we present a DFTAIMD study of liquid water using several GGA functionals as well as the van der Waals density functional (vdW-DF) of Dion et al. [Phys. Rev. Lett. 92, 246401(2004)]. As expected, we find that the density of water is grossly underestimated by GGA functionals. When a vdW-DF is used, the density improves drastically and the experimental diffusivity is reproduced without the need of thermal corrections. We analyze the origin of the density differences between all the functionals. We show that the vdW-DF increases the population of non-H-bonded interstitial sites, at distances between the first and second coordination shells. However, it excessively weakens the H-bond network, collapsing the second coordination shell. This structural problem is partially associated to the choice of GGA exchange in the vdW-DF. We show that a different choice for the exchange functional is enough to achieve an overall improvement both in structure and diffusivity.Comment: 11 pages, 9 figures, submitted. Revised versio

Fostering an EU strategy for security sector reform in the Mediterranean: learning from Turkish and Palestinian police reform experiences

In the absence of an overarching strategy, Turkey and the Palestinian territories depict how the EU has adopted different approaches to security sector reform (SSR) which have not facilitated the consolidation of a common EU foreign policy, though the situation might soon change given respective SSR-related documents from the Council and Commission. This report contributes to the security sector debate by stressing that in conflict and post-conflict scenarios endurable SSR requires fomenting synergies between the police and judicial sectors and the inclusion of DDR, in tandem with the institutional implementation of transparent, accountable and democratic oversight mechanisms. There is an adamant need for constructive consistency when applying this central facet of EU foreign policy in the Mediterranean basin and beyond. SSR is an emerging phenomenon in conflict, post-conflict and development scenarios that has acquired a prominent role within the policy agendas of key international actors. As a prelude to the two case studies, and in order to better understand the EU’s end goal, a brief analysis of the two pivotal European SSR documents is provided with particular emphasis on their contribution to develop a more coherent and effective EU presence in this field. More specific consideration is then given to the role played by SSR-related matters within the framework of two EU foreign policy mechanisms towards the Mediterranean: the Euro-Mediterranean Partnership (EMP), also known as the Barcelona Process, and the European Neighbourhood Policy (ENP). The two case studies - Turkey and the Palestinian territories - are exceptional due to the nature of their geopolitical position in regional and international politics. Both demonstrate different levels of state development and different degrees of monopoly over the legitimate use of force, which have contributed to the development of different EU approaches to SSR, tailored to the specific needs of the local context in which the Union is operating. Both cases also demonstrate how the EU emphasizes democracy or security depending on the respective circumstantial differences. Turkey exemplifies by and large the EU accession process of fulfilling democratic reform in return for EU membership. The Palestinian case illustrates how security demands precede democratic apertures. Together, both highlight the lack of a consistent and comprehensive EU SSR strategy, the lack of which continues to impede the emergence of a common EU foreign policy. This study makes a set of recommendations for the two case studies, and concludes with more general ones applicable to the broader Euro-Mediterranean area. These are mainly addressed to the European Commission, Council of the EU (and thus, Member States) and European Parliament. Moreover, the conclusions and recommendations included in this report could inspire the work of various advocacy groups in the fields of SSR, conflict resolution and democracy and human rights promotion given that, based on the “human security” logic, this report departs from the assumption that police reform encapsulates both modernising and democratising processes

Promoting Climate-Friendly Diets: What Should We Tell Consumers in Demark, Finland and France?

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Magnetic field morphology in nearby molecular clouds as revealed by starlight and submillimetre polarization

Within four nearby (d < 160 pc) molecular clouds, we statistically evaluate the structure of the interstellar magnetic field, projected on the plane of the sky and integrated along the line of sight, as inferred from the polarized thermal emission of Galactic dust observed by Planck at 353 GHz and from the optical and NIR polarization of background starlight. We compare the dispersion of the field orientation directly in vicinities with an area equivalent to that subtended by the Planck effective beam at 353 GHz (10') and using the second-order structure functions of the field orientation angles. We find that the average dispersion of the starlight-inferred field orientations within 10'-diameter vicinities is less than 20 deg, and that at these scales the mean field orientation is on average within 5 deg of that inferred from the submillimetre polarization observations in the considered regions. We also find that the dispersion of starlight polarization orientations and the polarization fractions within these vicinities are well reproduced by a Gaussian model of the turbulent structure of the magnetic field, in agreement with the findings reported by the Planck collaboration at scales greater than 10' and for comparable column densities. At scales greater than 10', we find differences of up to 14.7 deg between the second-order structure functions obtained from starlight and submillimetre polarization observations in the same positions in the plane of the sky, but comparison with a Gaussian model of the turbulent structure of the magnetic field indicates that these differences are small and are consistent with the difference in angular resolution between both techniques.Comment: 15 pages, 10 figures, submitted to A&

Dynamics of a map with power-law tail

We analyze a one-dimensional piecewise continuous discrete model proposed originally in studies on population ecology. The map is composed of a linear part and a power-law decreasing piece, and has three parameters. The system presents both regular and chaotic behavior. We study numerically and, in part, analytically different bifurcation structures. Particularly interesting is the description of the abrupt transition order-to-chaos mediated by an attractor made of an infinite number of limit cycles with only a finite number of different periods. It is shown that the power-law piece in the map is at the origin of this type of bifurcation. The system exhibits interior crises and crisis-induced intermittency.Comment: 28 pages, 17 figure

Variational finite-difference representation of the kinetic energy operator

A potential disadvantage of real-space-grid electronic structure methods is the lack of a variational principle and the concomitant increase of total energy with grid refinement. We show that the origin of this feature is the systematic underestimation of the kinetic energy by the finite difference representation of the Laplacian operator. We present an alternative representation that provides a rigorous upper bound estimate of the true kinetic energy and we illustrate its properties with a harmonic oscillator potential. For a more realistic application, we study the convergence of the total energy of bulk silicon using a real-space-grid density-functional code and employing both the conventional and the alternative representations of the kinetic energy operator.Comment: 3 pages, 3 figures, 1 table. To appear in Phys. Rev. B. Contribution for the 10th anniversary of the eprint serve

Fast diffusion of a Lennard-Jones cluster on a crystalline surface

We present a Molecular Dynamics study of large Lennard-Jones clusters evolving on a crystalline surface. The static and the dynamic properties of the cluster are described. We find that large clusters can diffuse rapidly, as experimentally observed. The role of the mismatch between the lattice parameters of the cluster and the substrate is emphasized to explain the diffusion of the cluster. This diffusion can be described as a Brownian motion induced by the vibrationnal coupling to the substrate, a mechanism that has not been previously considered for cluster diffusion.Comment: latex, 5 pages with figure

Robotic Spatial Printing

There has been significant research into large-scale 3D printing processes with industrial robots. These were initially used to extrude in a layered manner. In recent years, research has aimed to make use of six degrees of freedom instead of three. These so called "spatial extrusion" methods are based on a toolhead, mounted on a robot arm, that extrudes a material along a non horizontal spatial vector. This method is more time efficient but up to now has suffered from a number of limiting geometrical and structural constraints. This limited the formal possibilities to highly repetitive truss-like patterns. This paper presents a generalised approach to spatial extrusion based on the notion of discreteness. It explores how discrete computational design methods offer increased control over the organisation of toolpaths, without compromising design intent while maintaining structural integrity. The research argues that, compared to continuous methods, discrete methods are easier to prototype, compute and manufacture. A discrete approach to spatial printing uses a single toolpath fragment as basic unit for computation. This paper will describe a method based on a voxel space. The voxel contains geometrical information, toolpath fragments, that is subsequently assembled into a continuous, kilometers long path. The path can be designed in response to different criteria, such as structural performance, material behaviour or aesthetics. This approach is similar to the design of meta-materials - synthetic composite materials with a programmed performance that is not found in natural materials. Formal differentiation and structural performance is achieved, not through continuous variation, but through the recombination of discrete toolpath fragments. Combining voxel-based modelling with notions of meta-materials and discrete design opens this domain to large-scale 3D printing. Please write your abstract here by clicking this paragraph

Scholarly communication in transition: The use of question marks in the titles of scientific articles in medicine, life sciences and physics 1966–2005

The titles of scientific articles have a special significance. We examined nearly 20 million scientific articles and recorded the development of articles with a question mark at the end of their titles over the last 40 years. Our study was confined to the disciplines of physics, life sciences and medicine, where we found a significant increase from 50% to more than 200% in the number of articles with question-mark titles. We looked at the principle functions and structure of the titles of scientific papers, and we assume that marketing aspects are one of the decisive factors behind the growing usage of question-mark titles in scientific articles