Fitting in a complex chi^2 landscape using an optimized hypersurface sampling

Fitting a data set with a parametrized model can be seen geometrically as finding the global minimum of the chi^2 hypersurface, depending on a set of parameters {P_i}. This is usually done using the Levenberg-Marquardt algorithm. The main drawback of this algorithm is that despite of its fast convergence, it can get stuck if the parameters are not initialized close to the final solution. We propose a modification of the Metropolis algorithm introducing a parameter step tuning that optimizes the sampling of parameter space. The ability of the parameter tuning algorithm together with simulated annealing to find the global chi^2 hypersurface minimum, jumping across chi^2{P_i} barriers when necessary, is demonstrated with synthetic functions and with real data

Effects of internal molecular degrees of freedom on the thermal conductivity of some glasses and disordered crystals

The thermal conductivity κ(T) of the fully ordered stable phase II, the metastable phase III, the orientationally disordered (plastic) phase I, as well as the nonergodic orientational glass (OG) phase, of the glass former cyclohexanol (C 6H 11OH) has been measured under equilibrium vapor pressure within the 2-200 K temperature range. The main emphasis is here focused on the influence of the conformational disorder upon the thermal properties of this material. Comparison of results with those regarding cyanoclyclohexane (C 6H 11CN), a chemically related compound, serves to quantify the role played by the terminal groups -OH and -CN on the phonon scattering processes. The picture that emerges shows that motions of such groups do play a minor role as scattering centers, both within the low-temperature orientationally ordered phases as well as in the OG states. The results are analyzed within the Debye-Peierls relaxation time model for isotropic solids comprising mechanisms for long-wave phonon scattering within the OG and orientational ordered low-temperature phases, as well as others arising from localized short-wavelength vibrational modes as pictured by the Cahill-Pohl model. By means of complementary neutron and Raman scattering we show that in the OG state the energy landscapes for both compounds are very similar. © 2012 American Physical Society.This work was financially supported in part by the Spanish Ministry of Science and Innovation (Grant No. FIS2008-00837) and the Catalan Government (Grant No. 2009SGR-1251)Peer Reviewe

Thermal properties of halogen-ethane glassy crystals: Effects of orientational disorder and the role of internal molecular degrees of freedom

The thermal conductivity, specific heat, and specific volume of the orientational glass former 1,1,2-trichloro-1,2,2-trifluoroethane (CCl2F-CClF2, F-113) have been measured under equilibrium pressure within the low-temperature range, showing thermodynamic anomalies at ca. 120, 72, and 20 K. The results are discussed together with those pertaining to the structurally related 1,1,2,2-tetrachloro-1,2-difluoroethane (CCl2F-CCl2F, F-112), which also shows anomalies at 130, 90, and 60 K. The rich phase behavior of these compounds can be accounted for by the interplay between several of their degrees of freedom. The arrest of the degrees of freedom corresponding to the internal molecular rotation, responsible for the existence of two energetically distinct isomers, and the overall molecular orientation, source of the characteristic orientational disorder of plastic phases, can explain the anomalies at higher and intermediate temperatures, respectively. The soft-potential model has been used as the framework to describe the thermal properties at low temperatures. We show that the low-temperature anomaly of the compounds corresponds to a secondary relaxation, which can be associated with the appearance of Umklapp processes, i.e., anharmonic phonon-phonon scattering, that dominate thermal transport in that temperature rangeThis work was financially supported in part by the Spanish Ministry of Science and Innovation (Grant Nos. FIS2014-54734-P, FIS2011-23488, and MAT2014-57866- REDT), by the Catalan Government (Grant No. 2014SGR- 0581) and by the Comunidad de Madrid through program NANOFRONTMAG-CM (No. S2013/MIT-2850), as well as by the joint NAS Ukraine and Russian Foundation for Basic Research project “Metastable states of simple condensed systems” (Agreement No. N 7/-2013

Microscopic structures and dynamics of high- and low-density liquid trans-1,2-dichloroethylene

4 págs.; 3 figs.; PACS number s : 64.70.Ja, 61.05.fm, 61.20. p, 61.25.EmWe present a study of the dynamics and structural changes for trans-1,2-dichloroethylene between high- and low-density liquids using neutron-scattering techniques diffraction, small-angle neutron scattering, and time of flight spectroscopy and molecular-dynamics simulations. We show that changes in the short-range ordering of molecules goes along with a change in the molecular dynamics: both structure and dynamics of the highdensity liquid are more cooperative than those of the low-density liquid. The microscopic mechanism underlying the cooperative motions in the high-density liquid has been found to be related to the backscattering of molecules due to a strong correlation of molecular ordering. ©2010 The American Physical SocietyThis work was supported by the Spanish Ministry of Science and Technology Grants No. FIS2008-00837 and No. BES-2007-17418, by the Government of Catalonia Grant No. 2009SGR-1251 and by the European Commission MI3/FP7.Peer Reviewe

Red “Universidad, género, docencia e igualdad”

La Red de investigación en docencia universitaria “Universidad, docencia, genero e igualdad” persigue avanzar en la calidad e innovación de las enseñanzas universitarias a partir de la inclusión de la perspectiva de género. Se busca dar cumplimiento a las directrices generales de los nuevos planes de estudio respecto del principio de igualdad de oportunidades entre hombres y mujeres en la formación universitaria (Real Decreto 1393/2007. BOE nº 260, 30 de octubre de 2007). En la quinta edición de la Red, y dada su composición multidisciplinar, se ha trabajado en tres líneas de investigación: 1) mantenimiento del “Portal web con recursos docentes con perspectiva de género”, proyecto financiado por el Instituto de la Mujer (PACUI, 2012) e iniciado en el curso 2012-2013; 2) desarrollo de la primera versión de “iLengUA”, una herramienta informática para un discurso inclusivo e igualitario; y 3) diseño de una Guía de recomendaciones para la inclusión de la perspectiva de género en la docencia universitaria

Interplay between intramolecular and intermolecular structures of 1,1,2,2-tetrachloro-1,2-difluoroethane

We report on the interplay between the short-range order of molecules in the liquid phase of 1,1,2,2-tetrachloro-1,2-difluoroethane and the possible molecular conformations, trans and gauche. Two complementary approaches have been used to get a comprehensive picture: analysis of neutron-diffraction data by a Bayesian fit algorithm and a molecular dynamics simulation. The results of both show that the population of trans and gauche conformers in the liquid state can only correspond to the gauche conformer being more stable than the trans conformer. Distinct conformer geometries induce distinct molecular short-range orders around them, suggesting that a deep intra- and intermolecular interaction coupling is energetically favoring one of the conformers by reducing the total molecular free energy. © 2011 American Physical Society.This work was supported by the Spanish Ministry of Science and Innovation (Grant No. FIS2008-00837) and by the Government of Catalonia (Grant No. 2009SGR-1251).Peer Reviewe

Differences in first neighbor orientation behind the anomalies in the low and high density trans-1,2-dichloroethene liquid

7 pags., 10 figs., 2 tabs.Trans-1,2-dichloroethene (HClC=CClH) has several structural and dynamic anomalies between its low- and high-density liquid, previously found through neutron scattering experiments. To explain the microscopic origin of the differences found in those experiments, a series of molecular dynamics simulations were performed. The analysis of molecular short-range order shows that the number of molecules in the first neighbor shell is 12 for the high-density liquid and 11 for the low-density one. It also shows that the angular position of the center of mass of the first neighbor is roughly the same although the molecular orientation is not. In both liquids the first neighbor and its reference molecule arrange mainly in two configurations, each being the most probable in one of the liquids. First neighbors in the configuration that predominates in the high-density liquid tend to locate themselves closer to the reference molecule, an evidence that they are more strongly bonded. This arrangement facilitates a better packing of the rest of molecules in the first neighbor shell so that on average an additional molecule can be included, and is proposed to be the key in the explanation of all the observed anomalies in the characteristics of both liquids. © 2012 American Institute of Physics.This work was supported by the Spanish Ministry of Science and Innovation (FIS2008-00837) and by the Government of Catalonia (2009SGR-1251)

Disorder effects on heat transport properties of orientationally disordered crystals

6 págs.; 3 figs.; 2 tabs. ; PACS number s : 65.60. a, 66.70. f, 61.43. j, 63.50. xThe thermal conductivity k(T) of the orientational glass state of 1,2-difluoro-1,1,2,2-tetrachloroethane (CFCl2-CFCl2, Freon 112) and cyanocyclohexane (C6H11CN) has been measured under equilibrium pressure within the temperature range 2-100 K. The results show that the soft-potential model is able to account for low-temperature data and, indeed, quantitative agreement of all data considered is found within this realm. The details beyond such temperatures are heavily dependent on chemical details and a large plateau is observed for Freon 112 which is rationalized in terms of resonant scattering of phonons by simple oscillators. Such a view is given additional support by the presence of a strong low-frequency feature in the generalized frequency spectrum of the former material as proven by inelastic neutron-scattering spectroscopy. ©2010 The American Physical SocietyThis work was financially supported in part by the Spanish Ministry of Science and Innovation Grants No. FIS2008-00837 and No. MAT2007- 65711-C-4-01 and the Catalan Government Grant No. 2009SGR-1251 and joint projects of NAS of Ukraine and the Russian Foundation for Fundamental Research Grant No. N 9-2009.Peer Reviewe

Microscopic structures and dynamics of high- and low-density liquid trans-1,2-dichloroethylene

We present a study of the dynamics and structural changes for trans-1,2-dichloroethylene between high-and low-density liquids using neutron-scattering techniques (diffraction, small-angle neutron scattering, and time of flight spectroscopy) and molecular-dynamics simulations. We show that changes in the short-range ordering of molecules goes along with a change in the molecular dynamics: both structure and dynamics of the high-density liquid are more cooperative than those of the low-density liquid. The microscopic mechanism underlying the cooperative motions in the high-density liquid has been found to be related to the backscattering of molecules due to a strong correlation of molecular ordering