Recurrence quantification analysis of spatio-temporal chaotic transient in a closed unstirred Belousov-Zhabotinsky reaction

We analyse the transient spatio-temporal chaos that we observe in the Belousov-Zhabotinsky reaction performed in a closed unstirred batch reactor by recurrence quantification analysis (RQA). We characterize the chaotic transient by measuring the Lyapunov exponent and the Kaplan-Yorke dimension. The latter shows the fractality of the attractor. The importance of the coupling between hydrodynamics and kinetics for the onset of chaos is also shown

<i>Ab initio</i> based polarizable force field parametrization

Experimental and simulation studies of anion-water systems have pointed out the importance of molecular polarization for many phenomena ranging from hydrogen-bond dynamics to water interfaces structure. The study of such systems at molecular level is usually made with classical molecular dynamics simulations. Structural and dynamical features are deeply influenced by molecular and ionic polarizability, which parametrization in classical force field has been an object of long-standing efforts. Although when classical models are compared to ab initio calculations at condensed phase, it is found that the water dipole moments are underestimated by ~30%, while the anion shows an overpolarization at short distances. A model for chloride-water polarizable interaction is parametrized here, making use of Car-Parrinello simulations at condensed phase. The results hint to an innovative approach in polarizable force fields development, based on ab initio simulations, which do not suffer for the mentioned drawbacks. The method is general and can be applied to the modeling of different systems ranging from biomolecular to solid state simulations

Structural and dipolar fluctuations in liquid water: a Car–Parrinello molecular dynamics study

A Car–Parrinello molecular dynamics simulation was performed to investigate the local tetrahedral order, molecular dipole fluctuations and their interrelation with hydrogen bonding in liquid water. Water molecules were classified in three types, exhibiting low, intermediate and high tetrahedral order. Transitions from low to high tetrahedrally ordered structures take place only through transitions to the intermediate state. The molecular dipole moments depend strongly on the tetrahedral order and hydrogen bonding. The average dipole moment of water molecules with a strong tetrahedral order around them comes in excellent agreement with previous estimations of the dipole moment of ice Ih molecules.Postprint (author's final draft

Diffusion coefficient of ionic solvation shell molecules

It is shown that, for a tightly bound ion-solvation shell complex, the mean square displacement for solvation molecules is characterized by a long lasting transitory. This initial portion is related to the rotational relaxation of the complex and can reach up to several hundred picoseconds for a representative example such as the Mg2+ ion in water. As the diffusion coefficient is usually fitted using much shorter time spans, unnoticed overestimations are possible. It is argued that, instead of computing the aforementioned diffusion coefficient from the mean square displacement, it should be defined taking as a basic guideline the ratio between the rotational relaxation time of the complex and the lifetime within the first solvation shell

On the performance of molecular polarization methods. II. Water and carbon tetrachloride close to a cation

Our initial study on the performance of molecular polarization methods close to a positive point charge [M. Masia, M. Probst, and R. Rey, J. Chem. Phys. 121, 7362 (2004)] is extended to the case in which a molecule interacts with a real cation. Two different methods (point dipoles and shell model) are applied to both the ion and the molecule. The results are tested against high-level ab initio calculations for a molecule (water or carbon tetrachloride) close to Li+, Na+, Mg2+, and Ca2+. The monitored observable is in all cases the dimer electric dipole as a function of the ion-molecule distance for selected molecular orientations. The moderate disagreement previously obtained for point charges at intermediate distances, and attributed to the linearity of current polarization methods (as opposed to the nonlinear effects evident in ab initio calculations), is confirmed for real cations as well. More importantly, it is found that at short separations the phenomenological polarization methods studied here substantially overestimate the dipole moment induced if the ion is described quantum chemically as well, in contrast to the dipole moment induced by a point-charge ion, for which they show a better degree of accord with ab initio results. Such behavior can be understood in terms of a decrease of atomic polarizabilities due to the repulsion between electronic charge distributions at contact separations. It is shown that a reparametrization of the Thole method for damping of the electric field, used in conjunction with any polarization scheme, allows to satisfactorily reproduce the dimer dipole at short distances. In contrast with the original approach (developed for intramolecular interactions), the present reparametrization is ion and method dependent, and corresponding parameters are given for each case

Solvation dynamics and ion transport in conventional solvents and plasticizers

El argumento fundamental de esta tesis es el estudio de la solvatación iónica por medio de cálculos con ordenador. Tres lineas de investigación han sido seguidas:(i) Solvatación y mobilidad ionica. Las características principales del processo de intercambio entre la primera y la segunda capa de hidratación iónica para Li+ en agua se ha encontrado ser independiente del estado termodinamico en gran medida. Ha sido demostrado que el desplazamiento cuadrático medio de moléculas pertenecientes a complejos inertes está caracterizado por un largo transitorio debido a la lenta relajación rotacional del complejo. El incremento del coeficiente de difusión iónico debido a los intecambios en la capa de solvatación ha sido calculado por primera vez en el caso de Li+ y Na+. Finalmente, se han derivado leyes de probabilidad que ponen en relación la estereoquímica y la velocidad iónica instantanea.(ii) Plastificantes. Se propone un nuevo procedimiento para el desarrollo de campos de fuerza intramoleculares, que funciona satisfactoriamente en el caso de dos moleculas de interés en las Batterias a Iones de Litio: carbonato de etileno y &#61543;-butirolactona. Respecto a la solvatación de Li+ en los dos solventes, el ión está coordenado por 4 moleculas a través del oxigeno del carbonilo con pequeñas distorsiones de la geometría molecular. La nueva asignación de los modos vibracionales hecha para las dos moléculas ha permitido calcular los cambios inducidos por el ión litio, explicando varias caracteristicas de los espectros esperimentales.(iii) Polarización. Se ha estudiado la eficacia de los metodos de polarización más comunes para simulaciones de Dinámica Molecular en dímeros ión-molécula, usando calculos ab initio como referencia. En lugar de centrarnos en la superficie de energía potencial completa (procedimiento típico), se ha considerado solo la parte electrostática. Se han desarrollado nuevos modelos polarizables para agua y tetracloruro de carbono, que reproducen el comportamiento de sistemas carga-molécula. Ha sido encontrado que, en el caso de dímeros ión-molécula, se requiere una corrección de amortiguamento de la polarización a cortas distancias. El método de los dipolos puntuales junto al método de amortiguamento de Thole reproduce satisfactoriamente las características principales para cationes y aniones atómicos.The underlying topic of this thesis is the study of ion solvation by means of computer calculations. Three lines of investigation have been followed:(i) Solvation and Ionic Mobility. The main features of the exchange process between first and second ionic hydration shells for Li+ in water have been found to be largely independent of the thermodynamic state. It has been shown that the mean square displacement of molecules belonging to inert complexes is characterized by a long transient due to the slow rotational relaxation of the complex. The increase of the ionic diffusion coefficient due to solvation shell exchanges has been computed for the first time in the case of Li+ and Na+. Finally, probability laws have been derived which relate the stereochemistry and the instantaneous ionic velocity.(ii) Plasticizers. A new approach for the development of intramolecular force fields is proposed, which performs satisfactorily in the case of two molecules of interest for Lithium Ion Batteries: ethylene carbonate and &#61543;-butyrolactone. Concerning the solvation of Li+ in both solvents, it is coordinated by 4 molecules through the carbonyl oxygen with slight distorsions of the molecular geometry. The new vibrational mode assignment performed for both molecules has allowed to compute the vibrational shifts induced by the lithium ion, explaining a number of features present in the experimental spectra.(iii) Polarization. The performance of the most commonly used polarization methods for Molecular Dynamics simulation is studied for ion-molecule dimers, using ab initio calculations as benchmark. Instead of focusing on the full potential energy surface (the standard approach), only the electrostatic part is considered. New polarizable models have been developed for water and carbon tetrachloride, which reproduce the behaviour of charge-molecule systems. In the case of ion-molecule dimers it has been found that a polarization damping correction is required at short distances. The point dipole method in conjunction with the Thole damping scheme reproduces rather satisfactorily the main features both for atomic cations and anions

Recent advances in transient imaging: A computer graphics and vision perspective

Transient imaging has recently made a huge impact in the computer graphics and computer vision fields. By capturing, reconstructing, or simulating light transport at extreme temporal resolutions, researchers have proposed novel techniques to show movies of light in motion, see around corners, detect objects in highly-scattering media, or infer material properties from a distance, to name a few. The key idea is to leverage the wealth of information in the temporal domain at the pico or nanosecond resolution, information usually lost during the capture-time temporal integration. This paper presents recent advances in this field of transient imaging from a graphics and vision perspective, including capture techniques, analysis, applications and simulation

Recent advances in transient imaging: A computer graphics and vision perspective

Transient imaging has recently made a huge impact in the computer graphics and computer vision fields. By capturing, reconstructing, or simulating light transport at extreme temporal resolutions, researchers have proposed novel techniques to show movies of light in motion, see around corners, detect objects in highly-scattering media, or infer material properties from a distance, to name a few. The key idea is to leverage the wealth of information in the temporal domain at the pico or nanosecond resolution, information usually lost during the capture-time temporal integration. This paper presents recent advances in this field of transient imaging from a graphics and vision perspective, including capture techniques, analysis, applications and simulation

Bifurcations in spiral tip dynamics induced by natural convection in the Belousov–Zhabotinsky reaction

The transition to spatial-temporal complexity exhibited by spiral waves under the effect of gravitational field in the Belousov–Zhabotinsky reaction is numerically studied on the basis of spiral tip dynamics. Successive transformations in tip trajectories are characterized as a function of the hydrodynamical parameter and attributed to a Ruelle–Takens–Newhouse scenario to chaos. The analysis describes the emergence of complexity in terms of the interplay between the evolution of the velocity field and concentration waves. In particular, (i) by mapping the tip motion in relation to some hydrodynamical pseudopotentials, the general mechanism by which the velocity field affects the tip trajectory is pointed out, and, (ii) by comparing the dynamical evolutions of local and mean properties associated with the inhomogeneous structures and to the velocity field, a surprising correlation is found. The results suggest that the reaction-diffusion-convection (RDC) coupling addresses the system to some general regimes, whose nature is imposed by the hydrodynamical contribution. More generally, RDC coupling would be formalized as the phenomenon that governs the system and drives it to chaos