Simulation of quantum zero-point effects in water using a frequency-dependent thermostat

Molecules like water have vibrational modes with a zero-point energy well above room temperature. As a consequence, classical molecular dynamics simulations of their liquids largely underestimate the energy of modes with a higher zero-point temperature, which translates into an underestimation of covalent interatomic distances due to anharmonic effects. Zero-point effects can be recovered using path integral molecular dynamics simulations, but these are computationally expensive, making their combination with ab initio molecular dynamics simulations a challenge. As an alternative to path integral methods, from a computationally simple perspective, one would envision the design of a thermostat capable of equilibrating and maintaining the different vibrational modes at their corresponding zero-point temperatures. Recently, Ceriotti et al. (Phys. Rev. Lett. 102 020601 (2009)) introduced a framework to use a custom-tailored Langevin equation with correlated noise that can be used to include quantum fluctuations in classical molecular dynamics simulations. Here we show that it is possible to use the generalized Langevin equation with suppressed noise in combination with Nose-Hoover thermostats to efficiently impose a zero-point temperature on independent modes in liquid water. Using our simple and inexpensive method, we achieve excellent agreement for all atomic pair correlation functions compared to the path integral molecular dynamics simulation.Comment: 27 pages, 12 figs, Published versio

How Structural Are Structural Parameters?

This paper studies how stable over time are the so-called "structural parameters" of dynamic stochastic general equilibrium (DSGE) models. To answer this question, we estimate a medium-scale DSGE model with real and nominal rigidities using U.S. data. In our model, we allow for parameter drifting and rational expectations of the agents with respect to this drift. We document that there is strong evidence that parameters change within our sample. We illustrate variations in the parameters describing the monetary policy reaction function and in the parameters characterizing the pricing behavior of firms and households. Moreover, we show how the movements in the pricing parameters are correlated with inflation. Thus, our results cast doubts on the empirical relevance of Calvo models.

Comparing dynamic equilibrium economies to data

This paper studies the properties of the Bayesian approach to estimation and comparison of dynamic equilibrium economies. Both tasks can be performed even if the models are nonnested, misspecified, and nonlinear. First, the authors show that Bayesian methods have a classical interpretation: asymptotically the parameter point estimates converge to their pseudotrue values, and the best model under the Kullback-Leibler will have the highest posterior probability. Second, they illustrate the strong small sample behavior of the approach using a well-known application: the U.S. cattle cycle. Bayesian estimates outperform maximum likelihood results, and the proposed model is easily compared with a set of BVARs.Econometric models

Estimating dynamic equilibrium economies: linear versus nonlinear likelihood

This paper compares two methods for undertaking likelihood-based inference in dynamic equilibrium economies: a sequential Monte Carlo filter proposed by Fernández-Villaverde and Rubio-Ramírez (2004) and the Kalman filter. The sequential Monte Carlo filter exploits the nonlinear structure of the economy and evaluates the likelihood function of the model by simulation methods. The Kalman filter estimates a linearization of the economy around the steady state. The authors report two main results. First, both for simulated and for real data, the sequential Monte Carlo filter delivers a substantially better fit of the model to the data as measured by the marginal likelihood. This is true even for a nearly linear case. Second, the differences in terms of point estimates, even if relatively small in absolute values, have important effects on the moments of the model. The authors conclude that the nonlinear filter is a superior procedure for taking models to the data.

On the solution of the growth model with investment-specific technological change

Recent work by Greenwood, Hercowitz, and Krusell (1997 and 2000) and Fisher (2003) has emphasized the importance of investment-specific technological change as a main driving force behind long-run growth and the business cycle. This paper shows how the growth model with investment-specific technological change has a closed-form solution if capital fully depreciates. This solution furthers our understanding of the model, and it constitutes a useful benchmark to check the accuracy of numerical procedures to solve dynamic macroeconomic models in cases with several state variables.

El cambio lingüístico. Sus causas, mecanismos y consecuencias. José Luis Mendívil Giró. Madrid: Síntesis, 332 pages. ISBN: 9788490772102

Obra ressenyada: José Luis MENDÍVIL GIRÓ, El cambio lingüístico. Sus causas, mecanismos y consecuencias. Madrid: Síntesis, 2015

Construccionismo, postmodernismo y teoría de la evaluación. La función estratégica de la evaluación

La evaluación es una empresa lucrativa y un perfil profesional al que muchos se han sumado careciendo de las mínimas competencias y conocimientos exigibles. Evaluar es emitir juicios de valor sobre objetos diversos, incluidas las iniciativas de intervención social. El planteamiento convencional es racionalista (diagnóstico, planificación, intervención y análisis del impacto), a pesar de que los modelos racionalistas hayan perdido vigor en las ciencias sociales a favor de alternativas políticas, simbólicas o caóticas, tal como apreciamos en la historia del pensamiento organizacional. En una perspectiva construccionista, observamos además el poder reificador de los criterios escogidos en cada situación de evaluación. La evaluación sugiere o impone qué es lo correcto en cada caso; criterios que pasan a acotar, dar sentido y prioridad a determinado modo de entender las relaciones sociales, los objetivos o el modo de comportamiento personal y organizacional. Mi propuesta es elevar esta inevitable consecuencia práctica de las evaluaciones a rango de virtud política y ética, al servicio de los intereses y deseos de futuro de los grupos que participan en procesos evaluativos, primando su función estratégica sobre las tradicionales funciones de mejora y rendición de cuentas."Testing" and "evaluating" are money-making activities that offer a rewarding professional career; perhaps it is no surprise that the field includes numbers of self-styled "evaluators" who provide services without the required training. Evaluation means the systematic application of given criteria for judging the merit or value of a variety of 'assessables' - capacities, powers and services, including social policy interventions. The conventional perspective is rationalist (in diagnosis, planning, intervention and impact analysis), even though rationalism have lost their pre-eminence in the social sciences, in favour of political, symbolic or chaotic alternatives, as the history of organizational thought reveals. Alternatively, from a constructionist perspective, we see the power of evaluation criteria to reify what they ostensibly assess. Evaluations suggest or impose what is right; criteria of value define, give meaning and prioritize just one way of understanding social relations, program goals or individual and organizational behaviour. The suggestion in this article is to make a positive virtue of this inevitable pragmatic consequence of evaluation: to openly embrace it as an agent of change, emphasizing a strategical function beyond the traditional purposes of enhancement and accountability

Electrochemical and photo-electrochemical processes of Methylene blue oxidation by Ti/TiO2 electrodes modified with Fe-allophane

Indexación: Scopus.This work reports the degradation of methylene blue (MB) on Ti/TiO2 and Ti/TiO2/Fe-allophane electrodes in a pH 3 using 0.1 M Na2SO4 as support electrolyte. SEM micrographs show a homogeneous distribution of TiO2 over the whole electrode surface forming nanotubes and nanopores. Fe-allophane modified electrode shows the formation of large-grains agglomerate on the electrode surface due to allophane, which provides a greater surface area to the electrode due to meso and micropore structures. Preliminary cyclic voltammetry show that Ti/TiO2 has the typical voltammetric response due to Ti(III)/Ti(IV) pair. Diffusional problems were observed through of the film when the electrode is modified with Fe-allophane modifying the quasi-reversible process Ti(III)/Ti(IV). Different kind of methodologies in the degradation process were used: Electrochemistry (EC), Photochemistry (PC), Photoelectrochemistry (PEC) and Adsorption (Ads). These methods were developing to discard any reaction or interaction that is not of interest. On Ti/TiO2 with PC and Ads methodologies was not observed any activity to MB degradation showing that is not photosensitive and that the interaction between this and surface electrode is low. But with EC and PEC degradation to 55% is reached after 3 hours of electrolysis. With Ti/TiO2-Fe-allophane electrodes are observed a higher activity for all methodologies. The PC and Ads methods show that the MB degradation reaches to ∼20 % of the initial concentration. As mentioned above, the PC and Ads processes no show degradation on Ti/TiO2, therefore the degradation it only due to the adsorption of MB in/on allophane coat behaving as concentrator matrix. A lower improvement is observed with EC process when is incorporated Ti/TiO2-Fe-allophane is due to the barrier of the electrode surface by oxidation products. With PEC is reached the higher degradation value of ∼88 %, showing an improvement of the degradation with the presence of Fe-allophane. The results indicate that the main role of Fe-allophane on the electrode is similar to a concentrator matrix.http://ref.scielo.org/shz7t