Microscopic calculations of Hugoniot curves of neat TATB and of its detonation products

We compute the Hugoniot curves of both neat TATB and its detonation products mixture using atomistic simulation tools. To compute the Hugoniot states, we adapted our "Sampling Constraints in Average" (SCA) method (Maillet et al., Applied Math. Research eXpress 2008, 2009) to Monte-Carlo simulations. For neat TATB, we show that the potential proposed by Rai (Rai et al., J. Chem. Phys. 129, 2008) is not accurate enough to predict the Hugoniot curve and requires some optimization of its parameters. Concerning detonation products, thermodynamic properties at chemical equilibrium are computed using a specific RxMC method (Bourasseau et al., Phys. Chem. Chem. Phys. 13, 2011) taking into account the presence of carbon clusters in the fluid mixture. We show that this explicit description of the solid phase immersed in the fluid phase modifies the chemical equilibrium

New Potential Model for Molecular Dynamic Simulation of liquid HF. II -Parameter Optimization for Repulsion-Dispersion potential

International audienceIn order to build a complete potential model to perform classical molecular dynamic simulations of liquid HF, a new optimization method is proposed to obtain transferable parameters for repulsion-dispersion potential on the basis of ab initio reference data. This process is decomposed into two steps. The first step, using the force-matching method, consists in exploring the parameter space and selecting a first potential used as a start point for the second step. This last step consists in optimizing the parameters of the selected potential in order to reproduce reference thermodynamic and structural data. The obtained potential correctly reproduces the radial distribution functions and the pressures of HF liquid over a large range of thermodynamic states

Molecular Simulations of Hugoniots of detonation products mixtures at chemical equilibrium: Microscopic calculation of the Chapman-Jouguet State

International audienceIn this work, we used simultaneously the Reaction Ensemble Monte Carlo (ReMC) method and the Adaptive Erpenbeck Equation Of State (AE-EOS) method to directly calculate the thermodynamical and chemical equilibrium of mixtures of detonation products on the Hugoniot curve. The ReMC method (W. R. Smith and B. Triska, J. Chem. Phys. 100, pp 3019-3027 (1994)) allows to reach the chemical equilibrium of a reacting mixture, and the AE-EOS method (J. J. Erpenbeck, Phys. Rev. A, 46, p 6406 (1992)) constrains the system to satisfy the Hugoniot relation. Once the Hugoniot curve of the detonation products mixture is established, the CJ state of the explosive can be determined. Performing a NPT simulation at P(CJ) , T(CJ) , we then calculate the direct thermodynamic properties and the following derivative properties of the system using a fluctuation method: calorific capacities, sound velocity and Gruneisen coefficient. As the composition fluctuates, and the number of particles is not necessarily constant in this ensemble, a fluctuation formula has been developed to take into account the fluctuations of mole number and composition. This type of calculation has been applied to several usual energetic materials: nitromethane, tetranitromethane, hexanitroethane, PETN and RDX

Potential optimization for the calculation of shocked liquid nitromethane properties

International audienceWe present the results of the optimization of a classical molecular force field used to calculate the properties of shocked nitromethane by Monte Carlo simulations. The optimization technique allows a good transferability of the potential parameters on a broad range of thermodynamic conditions (temperature and pressure) since a large variety of reference data can be used in the optimization procedure, including densities, vaporization enthalpies or pressures along the Hugoniot curve. Results of calculated properties of shocked nitromethane are in good agreement with experimental shock hugoniot data, including temperature measurements of second shock hugoniot

Molecular Simulations of Shock to Detonation Transition in Nitromethane

An extension of the model described in a previous work of Maillet, Soulard and Stoltz based on a Dissipative Particule Dynamics is presented and applied to liquid nitromethane. Large scale non-equilibrium simulations of reacting nitromethane under sustained shock conditions allow a better understanding of the shock-to-detonation transition in homogeneous explosives. Moreover, the propagation of the reactive wave appears discontinuous since ignition points in the shocked material can be activated by the compressive waves emitted from the onset of chemical reactions

Effect of cationic chemical disorder on defect formation energies in uranium-plutonium mixed oxides

At the atomic scale, uranium-plutonium mixed oxides (U,Pu)O_2 are characterized by cationic chemical disorder, which entails that U and Pu cations are randomly distributed on the cation sublattice. In the present work, we study the impact of disorder on point-defect formation energies in (U,Pu)O_2 using interatomic-potential and Density Functional Theory (DFT+U) calculations. We focus on bound Schottky defects (BSD) that are among the most stable defects in these oxides. As a first step, we estimate the distance R_D around the BSD up to which the local chemical environment significantly affects their formation energy. To this end, we propose an original procedure in which the formation energy is computed for several supercells at varying levels of disorder. We conclude that the first three cation shells around the BSD have a non-negligible influence on their formation energy (R_{D} = 7.0 \{AA}). We apply then a systematic approach to compute the BSD formation energies for all the possible cation configurations on the first and second nearest neighbor shells around the BSD. We show that the formation energy can range in an interval of 0.97 eV, depending on the relative amount of U and Pu neighboring cations. Based on these results, we propose an interaction model that describes the effect of nominal and local composition on the BSD formation energy. Finally, the DFT+U benchmark calculations show a satisfactory agreement for configurations characterized by a U-rich local environment, and a larger mismatch in the case of a Pu-rich one. In summary, this work provides valuable insights on the properties of BSD defects in (U,Pu)O_2, and can represent a valid strategy to study point defect properties in disordered compounds.Comment: 33 pages, 20 figure

Constant entropy sampling and release waves of shock compressions

We present several equilibrium methods that allow to compute isentropic processes, either during the compression or the release of the material. These methods are applied to compute the isentropic release of a shocked monoatomic liquid at high pressure and temperature. Moreover, equilibrium results of isentropic release are compared to the direct nonequilibrium simulation of the same process. We show that due to the viscosity of the liquid but also to nonequilibrium effects, the release of the system is not strictly isentropic

Prédiction des propriétés d'équilibre de phases par simulation moléculaire (développement d'algorithmes et optimisation de potentiels)

La simulation moléculaire est en passe de devenir un moyen usuel de détermination des propriétés d'équilibre de phases des fluides. En effet, d'importantes améliorations ont été récemment apportées en matière de méthodologie, et plusieurs algorithmes efficaces sont désormais disponibles pour obtenir la courbe de coexistence liquide-vapeur d'un fluide. Cependant, malgré ces importants progrès, la prédiction par simulation moléculaire n'est pas encore couramment employée dans le domaine industriel. S'inscrivant dans un projet de recherche du CNRS en collaboration avec l'Institut Français du Pétrole, l'objectif de ce travail de thèse a été de développer un ensemble d'outils basés sur la simulation moléculaire permettant la prédiction des propriétés d'équilibre de phases d'une grande partie des fluides d'hydrocarbures. A partir du programme GIBBS déjà existant au laboratoire de Chimie Physique, et qui permettait la simulation de petites molécules rigides, et d'alcanes linéaires et branchés, plusieurs sous programmes ont été implantés permettant la simulation de molécules cycliques, semi-rigides, et ramifiées. Du point de vue des potentiels, une procédure originale d'optimisation des paramètres de potentiels intermoléculaires a été développée et testée. Cette méthode, exploitant les avantages d'une simulation Monte Carlo, permet l'optimisation rapide de paramètres témoignant d'un réalisme physique important, et transférables entre molécules du même type. Ce travail a permis l'établissement de potentiels de type Lennard Jones pour la simulation des alcanes cycliques, des oléfines, et des alcools. Finalement, de nombreuses simulations prédictives et fiables ont pu être effectuées, notamment concernant les alcanes cycliques, les oléfines, les alcools, et le pristane. L'ensemble des résultats de simulation apparaît en excellent accord avec les résultats expérimentaux déjà existants.The use of molecular simulation techniques is becoming a routine way to predict phase equilibrium properties of fluids. Indeed, in recent years, important improvements have been made in simulation methodology and several efficient algorithms are now available to obtain a liquid-vapour coexistence curve. Nevertheless, despite those real progresses, property prediction is not yet a standard practice in industrial applications. Consequently, this PhD work is included in an important research program developed by the CNRS, in collaboration with the Institut Français du Pétrole, which aim is to predict equilibrium properties of all hydrocarbon molecules through molecular simulation techniques, and more precisely using the Monte Carlo method. Starting from the Gibbs code, already developed at the Laboratory of Physical Chemistry, which allows the simulation of small rigid molecules and linear and branched alkanes, several subroutines have been implemented to extend its use to cyclic molecules, semi-rigid molecules and long branched molecules. Moreover, a global original optimisation method is proposed and validated. This method, based on the advantages given by Monte Carlo methods, allows the efficient optimisation of realistic parameters transferable to other molecules in the same family. This work has allowed the optimisation of Lennard Jones parameters of cyclic alkanes, olefins, and alcohols. Finally, numerous simulations have been performed to predict equilibrium properties of cyclic alkanes, olefins, alcohols and pristane. All simulation results appear in very good accordance with available experimental results.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Surface Tension and Long Range Corrections of Cylindrical Interfaces

International audienceThe calculation of the surface tension of curved interfaces has been deeply investigated from molecular simulation during this last past decade. Recently, the thermodynamic Test-Area (TA) approach has been extended to the calculation of surface tension of curved interfaces. In the case of the cylindrical air-liquid interfaces of water and Lennard-Jones (LJ) fluids, it was shown that the surface tension was independent of the curvature of the interface. In addition, the surface tension of the cylindrical interface is higher than that of the planar interface. Molecular simulations of cylindrical interfaces have been so far performed i) by using a shifted potential ii) by means of large cutoff without periodic boundary conditions or iii) by ignoring the long range corrections (LRC) to the surface tension due to the difficulty to estimate them. Indeed, unlike the planar interfaces there are no available operational expressions to consider the tail corrections to the surface tension of cylindrical interfaces. We propose here to develop the long range corrections of the surface tension for cylindrical interfaces by using the non exponential TA method (TA2). We also extend the formulation of the Mecke-Winkelmann corrections initially developed for planar surfaces to cylindrical interfaces. We complete this study by the calculation of the surface tension of cylindrical surfaces of liquid tin and copper using the embedded atom model (EAM) potentials