arXiv subjects

Jean-Bernard Maillet

Publications and source records attributed to Jean-Bernard Maillet.

17 recordsLinked to original sources

arXiv2026

Fully anharmonic calculations of the free energy of migration of point defects in UO2 and PuO2

Calculating diffusion rates of point defects in materials typically relies on the harmonic approximation to estimate migration free energies. However, anharmonic effects can have a large impact on diffusion properties, and explicitly accounting for them is usually computationally demanding and difficult to achieve in practice. In this work, we investigate the role of anharmonic effects on defect migration in UO2 and PuO2 using the potential of average force integration (PAFI) method. Fully anharmonic migration free energies are computed for several cation and anion defect types, using the Cooper-Rushton-Grimes (CRG) potential and a recently developed machine learning spectral neighbour analysis potential (SNAP) for UO2. Results are systematically compared to harmonic estimates based on attempt frequencies and the Debye approximation. Our results reveal that the validity of the harmonic approximation strongly depends on the defect type and the underlying potential, with significant deviations observed in several cases. In particular, defect migration barriers are found to decrease strongly with increasing temperature (up to 1 eV between 0 and 1200 K), and anharmonic contributions can substantially modify migration entropies and, consequently, diffusion coefficients. Comparing defect migration in UO2 and PuO2 using the CRG potential reveals that PuO2 has lower migration enthalpies at 0~K for all considered defects, but this is compensated by higher attempt frequencies, resulting in similar overall jump frequencies in UO2 and PuO2. These findings provide insight into the limitations of commonly used approximations and highlight the importance of anharmonic effects for predictive modeling of diffusion in nuclear fuels as well as in other classes of materials.

cond-mat.mtrl-sci↗

arXiv2025

Efforts in Modeling the Mechanics and Chemistry of Energetic Materials Across Scales

Recent developments dedicated to the building of multiscale mechanical and chemical constitutive laws for energetic molecular crystals are presented and discussed. In particular, various tools have been specifically incorporated in molecular dynamics codes to facilitate the subsequent information transfer to the continuum, i.e. finite elements simulation codes. Atomistic simulations have been augmented with the capability to follow specific deformation paths as well as local Lagrangian mechanical metrics, enabling the computation of materials flow stress surface. This mechanistic library allowed the construction of a comprehensive non-linear hyperelastic continuum model including crystal plasticity and twinning for TATB. Besides, recent advances in analyzing reactive molecular dynamics simulations with unsupervised learning algorithms has enabled the identification and calibration of chemical decomposition kinetics for RDX and TATB single crystal. In the present work, the procedure is applied to $\beta$-HMX and extended with the calibration of a multi-components equation of state. These two ingredients are implemented in a finite-element code in order to model the shock-to-detonation transition at the mesoscale level and to study dimensionality effects in quasi-static hotspots. Finally, these dedicated efforts towards a comprehensive multiscale modeling of explosives has also given rise to the need for new prospective experiments, discussed throughout the paper.

cond-mat.mes-hall↗

arXiv2025

Extracting flow stress surfaces of pristine materials using deformation paths in MD simulations

Accurate simulation of deformation processes at the atomic scale is critical for predicting the mechanical response of materials and particularly the calculation of directional flow stresses. This work presents a method for applying arbitrary deformation paths in LAMMPS while adhering to its convention that supercell periodic vectors a, b are aligned such that a coincides with the x-axis and b lies in the (x,y) plane. This method is particularly relevant for materials with low crystal symmetry and also for exploring non uniaxial deformations. The first step of the method consists in generating the simulation frame tensor's time evolution upon any deformation, which may initially violate LAMMPS alignment constraints. This constraint is then overcome by the application of a rigid body rotation to realign the tensor with LAMMPS's convention, ensuring valid periodic boundary conditions. The resulting lengths and tilt factors from the rotated tensor are expressed analytically using third-order polynomials and applied to the simulation cell using the fix deform command. The present approach versatility is validated with the calculation of directional flow stresses for various materials upon constant volume shear, tension and compression, demonstrating its effectiveness in simulations involving complex deformation scenarios and diverse crystal structures. The flow stress surface extracted from these simulations are finally analyzed as the fingerprint of all deformation mechanisms occurring in the material.

cond-mat.mtrl-sci↗

arXiv2023

Neighbors Map: an Efficient Atomic Descriptor for Structural Analysis

Accurate structural analysis is essential to gain physical knowledge and understanding of atomic-scale processes in materials from atomistic simulations. However, traditional analysis methods often reach their limits when applied to crystalline systems with thermal fluctuations, defect-induced distortions, partial vitrification, etc. In order to enhance the means of structural analysis, we present a novel descriptor for encoding atomic environments into 2D images, based on a pixelated representation of graph-like architecture with weighted edge connections of neighboring atoms. This descriptor is well adapted for Convolutional Neural Networks and enables accurate structural analysis at a low computational cost. In this paper, we showcase a series of applications, including the classification of crystalline structures in distorted systems, tracking phase transformations up to the melting temperature, and analyzing liquid-to-amorphous transitions in pure metals and alloys. This work provides the foundation for robust and efficient structural analysis in materials science, opening up new possibilities for studying complex structural processes, which can not be described with traditional approaches.

cond-mat.mtrl-sci↗

arXiv2023

Robust crystal structure identification at extreme conditions using a density-independent spectral descriptor and supervised learning

The increased time- and length-scale of classical molecular dynamics simulations have led to raw data flows surpassing storage capacities, necessitating on-the-fly integration of structural analysis algorithms. As a result, algorithms must be computationally efficient, accurate, and stable at finite temperature to reliably extract the relevant features of the data at simulation time. In this work, we leverage spectral descriptors to encode local atomic environments and build crystal structure classification models. In addition to the classical way spectral descriptors are computed, i.e. over a fixed radius neighborhood sphere around a central atom, we propose an extension to make them independent from the material's density. Models are trained on defect-free crystal structures with moderate thermal noise and elastic deformation, using the linear discriminant analysis (LDA) method for dimensionality reduction and logistic regression (LR) for subsequent classification. The proposed classification model is intentionally designed to be simple, incorporating only a limited number of parameters. This deliberate simplicity enables the model to be trained effectively even when working with small databases. Despite the limited training data, the model still demonstrates inherent transferability, making it applicable to a broader range of scenarios and datasets. The accuracy of our models in extreme conditions is compared to traditional algorithms from the literature, namely adaptive common neighbor analysis (a-CNA), polyhedral template matching (PTM) and diamond structure identification (IDS). Finally, we showcase two applications of our method: tracking a solid-solid BCC-to-HCP phase transformation in Zirconium at high pressure up to high temperature, and visualizing stress-induced dislocation loop expansion in single crystal FCC Aluminum containing a Frank-Read source, at high temperature.

cond-mat.mtrl-sci↗

arXiv2022

Elastic anisotropy of 1,3,5-Triamino-2,4,6-Trinitrobenzene as a function of temperature and pressure: A Molecular Dynamics study

The equation of state of the triclinic compound 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) as well as its second-order isothermal elastic tensor were computed through classical molecular dynamics simulations under various temperature and pressure conditions. Hydrostatic pressures similar to previous diamond anvil cell experiments were imposed within the range [0, 66] GPa and temperatures chosen between 100 and 900 K in conjunction with the most recent version of an all-atom fully-flexible molecule force field. The isothermal elastic constants were computed using the generalized Hooke's law by fitting Cauchy stress vs. linear strain curves. Along isobaric pathways, TATB single crystal stiffness is found to undergo linear softening, less pronounced at high pressure, while maintaining its elastic anisotropy. On the other hand, along an isothermal pathways, a non-linear increase is observed in the elastic constants with a significant decrease in anisotropy. Towards a precise mesoscopic modeling of TATB single crystal mechanical behavior, we provide "ready to plug-in" analytical formulations of the P,V,T equation of state and pressure-temperature dependent non-linear elasticity.

cond-mat.mtrl-sci↗

arXiv2021

Quantum-accurate magneto-elastic predictions with classical spin-lattice dynamics

A data-driven framework is presented for building magneto-elastic machine-learning interatomic potentials (ML-IAPs) for large-scale spin-lattice dynamics simulations. The magneto-elastic ML-IAPs are constructed by coupling a collective atomic spin model with an ML-IAP. Together they represent a potential energy surface from which the mechanical forces on the atoms and the precession dynamics of the atomic spins are computed. Both the atomic spin model and the ML-IAP are parametrized on data from first-principles calculations. We demonstrate the efficacy of our data-driven framework across magneto-structural phase transitions by generating a magneto-elastic ML-IAP for α-iron. The combined potential energy surface yields excellent agreement with first-principles magneto-elastic calculations and quantitative predictions of diverse materials properties including bulk modulus, magnetization, and specific heat across the ferromagnetic-paramagnetic phase transition.

cond-mat.mtrl-sci↗

arXiv2020

Machine learning force fields and coarse-grained variables in molecular dynamics: application to materials and biological systems

Machine learning encompasses a set of tools and algorithms which are now becoming popular in almost all scientific and technological fields. This is true for molecular dynamics as well, where machine learning offers promises of extracting valuable information from the enormous amounts of data generated by simulation of complex systems. We provide here a review of our current understanding of goals, benefits, and limitations of machine learning techniques for computational studies on atomistic systems, focusing on the construction of empirical force fields from ab-initio databases and the determination of reaction coordinates for free energy computation and enhanced sampling.

physics.comp-ph↗

arXiv2017

Simulations of detonation waves with smoothed dissipative particle dynamics

Smoothed Dissipative Particle Dynamics (SDPD) is a mesoscopic method which allows to select the level of resolution at which a fluid is simulated. The aim of this work is to extend SDPD to chemically reactive systems.To this end, an additional progress variable is attached to each mesoparticle and evolves according to chemical kinetics. This reactive SDPD model is illustrated with numerical studies of the shock-to-detonation transition in nitromethane as well as the stationary behavior of the reactive wave.

physics.flu-dyn↗

arXiv2015

Permutation-invariant distance between atomic configurations

We present a permutation-invariant distance between atomic configurations, defined through a functional representation of atomic positions. This distance enables to directly compare different atomic environments with an arbitrary number of particles, without going through a space of reduced dimensionality (i.e. fingerprints) as an intermediate step. Moreover, this distance is naturally invariant through permutations of atoms, avoiding the time consuming associated minimization required by other common criteria (like the Root Mean Square Distance). Finally, the invariance through global rotations is accounted for by a minimization procedure in the space of rotations solved by Monte Carlo simulated annealing. A formal framework is also introduced, showing that the distance we propose verifies the property of a metric on the space of atomic configurations. Two examples of applications are proposed. The first one consists in evaluating faithfulness of some fingerprints (or descriptors), i.e. their capacity to represent the structural information of a configuration. The second application concerns structural analysis, where our distance proves to be efficient in discriminating different local structures and even classifying their degree of similarity.

cond-mat.mtrl-sci↗

arXiv2013

Local density dependent potential for compressible mesoparticles

We focus on finding a coarse grained description able to reproduce the thermodynamic behavior of a molecular system by using mesoparticles representing several molecules. Interactions between mesoparticles are modelled by an interparticle potential, and an additional internal equation of state is used to account for the thermic contribution of coarse grained internal degrees of freedom. Moreover, as strong non-equilibrium situations over a wide range of pressure and density are targeted, the internal compressibility of these mesoparticles has to be considered. This is done by introducing a dependence of the potential on the local environment of the mesoparticles, either by defining a spherical local density or by means of a Voronoi tessellation. As an example, a local density dependent potential is fitted to reproduce the Hugoniot curve of a model of nitromethane, where each mesoparticle represents one thousand molecules.

cond-mat.soft↗

arXiv2013

Kinetics of heterogeneous nucleation and growth: An approach based on a grain explicit model

A model for phase transitions initiated on grain boundaries is proposed and tested against numerical simulations: this approach based on a grain explicit model (GEM) allows to consider the granular structure, yielding accurate predictions for a wide span of nucleation processes. Comparisons are made with classical models of homogeneous (JMAK) as well as heterogeneous (Cahn) nucleation. A transition scale based on material properties is proposed, allowing to discriminate between random and site saturated regimes. Finally, we discuss the relationship between an Avrami type exponent and the transition regime, drawing conditions for its extraction from experiments.

cond-mat.mtrl-sci↗

arXiv2011

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.

cond-mat.stat-mech↗

arXiv2009

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.

cond-mat.stat-mech↗

arXiv2008

Sampling constraints in average: The example of Hugoniot curves

We present a method for sampling microscopic configurations of a physical system distributed according to a canonical (Boltzmann-Gibbs) measure, with a constraint holding in average. Assuming that the constraint can be controlled by the volume and/or the temperature of the system, and considering the control parameter as a dynamical variable, a sampling strategy based on a nonlinear stochastic process is proposed. Convergence results for this dynamics are proved using entropy estimates.As an application, we consider the computation of points along the Hugoniot curve, which are equilibrium states obtained after equilibration of a material heated and compressed by a shock wave.

cond-mat.stat-mech↗

arXiv2007

A reduced model for shock and detonation waves. II. The reactive case

We present a mesoscopic model for reactive shock waves, which extends a previous model proposed in [G. Stoltz, Europhys. Lett. 76 (2006), 849]. A complex molecule (or a group of molecules) is replaced by a single mesoparticle, evolving according to some Dissipative Particle Dynamics. Chemical reactions can be handled in a mean way by considering an additional variable per particle describing a rate of reaction. The evolution of this rate is governed by the kinetics of a reversible exothermic reaction. Numerical results give profiles in qualitative agreement with all-atom studies.

cond-mat.mtrl-sci↗

arXiv2001

Three dimensional hysdrodynamic lattice-gas simulations of binary immiscible and ternary amphiphilic flow through porous media

We report the results of a study of multiphase flow in porous media. A Darcy's law for steady multiphase flow was investigated for both binary and ternary amphiphilic flow. Linear flux-forcing relationships satisfying Onsager reciprocity were shown to be a good approximation of the simulation data. The dependence of the relative permeability coefficients on water saturation was investigated and showed good qualitative agreement with experimental data. Non-steady state invasion flows were investigated, with particular interest in the asymptotic residual oil saturation. The addition of surfactant to the invasive fluid was shown to significantly reduce the residual oil saturation.

cond-mat.soft↗