scholarly journals Supercritical water anomalies in the vicinity of the Widom line

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Konstantinos Karalis ◽  
Christian Ludwig ◽  
Bojan Niceno
Keyword(s):  
Supercritical Water ◽  
Industrial Applications ◽  
Density Fluctuations ◽  
Anomalous Behaviour ◽  
Detailed Knowledge ◽  
Interatomic Potentials ◽  
Homogeneous Fluid ◽  
Widom Line ◽  
Scale Inhomogeneity ◽  
Dynamics Simulations

Abstract Supercritical water is used in a variety of chemical and industrial applications. As a consequence, a detailed knowledge of the structure-properties correlations is of uttermost importance. Although supercritical water was considered as a homogeneous fluid, recent studies revealed an anomalous behaviour due to nanoscale density fluctuations (inhomogeneity). The inhomogeneity is clearly demarked through the Widom line (maxima in response factions) and drastically affect the properties. In the current study the physical properties of supercritical water have been determined by classical molecular dynamics simulations using a variety of polarized and polarizable interatomic potentials. Their validity which was not available at supercritical conditions has been assessed based on the ability to reproduce experimental data. Overall, the polarized TIP4P/2005 model accurately predicted the properties of water in both liquid-like and gas-like regions. All interatomic potentials captured the anomalous behaviour providing a direct evidence of molecular-scale inhomogeneity.

scholarly journals Visualization of supercritical water pseudo-boiling at Widom line crossover

2019 ◽  
Vol 10 (1) ◽  
Author(s):  
Florentina Maxim ◽  
Cristian Contescu ◽  
Pierre Boillat ◽  
Bojan Niceno ◽  
Konstantinos Karalis ◽  
...  
Keyword(s):  
Supercritical Water ◽  
New Technologies ◽  
Waste Treatment ◽  
Imaging Techniques ◽  
Current Theory ◽  
Density Fluctuations ◽  
Neutron Imaging ◽  
Materials Synthesis ◽  
Natural Systems ◽  
Widom Line

Abstract Supercritical water is a green solvent used in many technological applications including materials synthesis, nuclear engineering, bioenergy, or waste treatment and it occurs in nature. Despite its relevance in natural systems and technical applications, the supercritical state of water is still not well understood. Recent theories predict that liquid-like (LL) and gas-like (GL) supercritical water are metastable phases, and that the so-called Widom line zone is marking the crossover between LL and GL behavior of water. With neutron imaging techniques, we succeed to monitor density fluctuations of supercritical water while the system evolves rapidly from LL to GL as the Widom line is crossed during isobaric heating. Our observations show that the Widom line of water can be identified experimentally and they are in agreement with the current theory of supercritical fluid pseudo-boiling. This fundamental understanding allows optimizing and developing new technologies using supercritical water as a solvent.

Heterogeneous character of supercritical water at 400 °C and different densities unveiled by simulation

RSC Advances ◽  
2016 ◽  
Vol 6 (36) ◽  
pp. 30484-30487 ◽  
Author(s):  
Noureddine Metatla ◽  
Fabien Lafond ◽  
Jean-Paul Jay-Gerin ◽  
Armand Soldera
Keyword(s):  
Molecular Dynamics ◽  
Molecular Dynamics Simulations ◽  
Supercritical Water ◽  
Dynamics Simulations ◽  
Clustering Behavior

Molecular dynamics simulations are used to examine the molecular microstructures and the “clustering” behavior of supercritical water at 400 °C and different densities.

scholarly journals Differentiable sampling of molecular geometries with uncertainty-based adversarial attacks

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Daniel Schwalbe-Koda ◽  
Aik Rui Tan ◽  
Rafael Gómez-Bombarelli
Keyword(s):  
Potential Energy Surfaces ◽  
Automatic Differentiation ◽  
Ground Truth ◽  
Training Data ◽  
Interatomic Potentials ◽  
Collective Variables ◽  
Molecule Interactions ◽  
Dynamics Simulations ◽  
Fast Prediction ◽  
Energy Surfaces

AbstractNeural network (NN) interatomic potentials provide fast prediction of potential energy surfaces, closely matching the accuracy of the electronic structure methods used to produce the training data. However, NN predictions are only reliable within well-learned training domains, and show volatile behavior when extrapolating. Uncertainty quantification methods can flag atomic configurations for which prediction confidence is low, but arriving at such uncertain regions requires expensive sampling of the NN phase space, often using atomistic simulations. Here, we exploit automatic differentiation to drive atomistic systems towards high-likelihood, high-uncertainty configurations without the need for molecular dynamics simulations. By performing adversarial attacks on an uncertainty metric, informative geometries that expand the training domain of NNs are sampled. When combined with an active learning loop, this approach bootstraps and improves NN potentials while decreasing the number of calls to the ground truth method. This efficiency is demonstrated on sampling of kinetic barriers, collective variables in molecules, and supramolecular chemistry in zeolite-molecule interactions, and can be extended to any NN potential architecture and materials system.

Molecular Dynamics Simulations of Atoms Diffusion in Solid

2018 ◽  
Vol 15 ◽  
pp. 51-64
Author(s):  
Yu Lu Zhou ◽  
Xiao Ma Tao ◽  
Qing Hou ◽  
Yi Fang Ouyang
Keyword(s):  
Molecular Dynamics ◽  
Diffusion Coefficients ◽  
Md Simulations ◽  
Einstein Relation ◽  
Interatomic Potentials ◽  
Body System ◽  
Many Body ◽  
Point Particles ◽  
Bulk Surface ◽  
Dynamics Simulations

Molecular dynamics (MD) simulations, which treat atoms as point particles and trace their individual trajectories, are always employed to investigate the transport properties of a many-body system. The diffusion coefficients of atoms in solid can be obtained by the Einstein relation and the Green-Kubo relation. An overview of the MD simulations of atoms diffusion in the bulk, surface and grain boundary is provided. We also give an example of the diffusion of helium in tungsten to illustrate the procedure, as well as the importance of the choice of interatomic potentials. MD simulations can provide intuitive insights into the atomic mechanisms of diffusion.

Solubility of Cellulose in Supercritical Water Studied by Molecular Dynamics Simulations

2015 ◽  
Vol 119 (13) ◽  
pp. 4739-4748 ◽  
Author(s):  
Lasse K. Tolonen ◽  
Malin Bergenstråhle-Wohlert ◽  
Herbert Sixta ◽  
Jakob Wohlert
Keyword(s):  
Molecular Dynamics ◽  
Molecular Dynamics Simulations ◽  
Supercritical Water ◽  
Dynamics Simulations

Application of Scaling to Multibody Dynamics Simulations

2015 ◽  
Author(s):  
William Prescott
Keyword(s):  
Equations Of Motion ◽  
Industrial Applications ◽  
Differential Algebraic Equations ◽  
Algebraic Equations ◽  
Step Size ◽  
Virtual Lab ◽  
Long Run ◽  
Multibody Dynamic ◽  
Dynamics Simulations ◽  
Stability Issues

This paper will examine the importance of applying scaling to the equations of motion for multibody dynamic systems when applied to industrial applications. If a Cartesian formulation is used to formulate the equations of motion of a multibody dynamic system the resulting equations are a set of differential algebraic equations (DAEs). The algebraic components of the DAEs arise from appending the joint equations used to model revolute, cylindrical, translational and other joints to the Newton-Euler dynamic equations of motion. Stability issues can arise in an ill-conditioned Jacobian matrix of the integration method this will result in poor convergence of the implicit integrator’s Newton method. The repeated failures of the Newton’s method will require a small step size and therefore simulations that require long run times to complete. Recent advances in rescaling the equations of motion have been proposed to address this problem. This paper will see if these methods or a variant addresses not only stability concerns, but also efficiency. The scaling techniques are applied to the Gear-Gupta-Leimkuhler (GGL) formulation for multibody problems by embedding them into the commercial multibody code (MBS) Virtual. Lab Motion and then use them to solve an industrial sized automotive example to see if performance is improved.

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