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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Flow-driven spectral chaos (FSC) method for simulating long-time dynamics of arbitrary-order non-linear stochastic dynamical systems

Uncertainty quantification techniques such as the time-dependent generalized polynomial chaos (TD-gPC) use an adaptive orthogonal basis to better represent the stochastic part of the solution space (aka random function space) in time. However, because the random function space is constructed using tensor products, TD-gPC-based methods are known to suffer from the curse of dimensionality. Here, we introduce a new numerical method called the flow-driven spectral chaos (FSC) which overcomes this curse of dimensionality at the random-function-space level. The proposed method is not only computationally more efficient than existing TD-gPC-based methods but is also far more accurate. The FSC method uses the concept of enriched stochastic flow maps to track the evolution of a finite-dimensional random function space efficiently in time. To transfer the probability information from one random function space to another, two approaches are developed and studied herein. In the first approach, the probability information is transferred in the mean-square sense, whereas in the second approach the transfer is done exactly using a new theorem that was developed for this purpose. The FSC method can quantify uncertainties with high fidelity, especially for the long-time response of stochastic dynamical systems governed by ODEs of arbitrary order. Six representative numerical examples, including a nonlinear problem (the Van-der-Pol oscillator), are presented to demonstrate the performance of the FSC method and corroborate the claims of its superior numerical properties. Finally, a parametric, high-dimensional stochastic problem is used to demonstrate that when the FSC method is used in conjunction with Monte Carlo integration, the curse of dimensionality can be overcome altogether.

(nonlinear) stochastic dynamical systems↗
DOE OSTI2020

Enabling Large-Scale Condensed-Phase Hybrid Density Functional Theory Based Ab Initio Molecular Dynamics. 1. Theory, Algorithm, and Performance

By including a fraction of exact exchange (EXX), hybrid functionals reduce the self-interaction error in semilocal density functional theory (DFT) and thereby furnish a more accurate and reliable description of the underlying electronic structure in systems throughout biology, chemistry, physics, and materials science. However, the high computational cost associated with the evaluation of all required EXX quantities has limited the applicability of hybrid DFT in the treatment of large molecules and complex condensed-phase materials. To overcome this limitation, we describe a linear-scaling approach that utilizes a local representation of the occupied orbitals (e.g., maximally localized Wannier functions (MLWFs)) to exploit the sparsity in the real-space evaluation of the quantum mechanical exchange interaction in finite-gap systems. In this work, we present a detailed description of the theoretical and algorithmic advances required to perform MLWF-based ab initio molecular dynamics (AIMD) simulations of large-scale condensed-phase systems of interest at the hybrid DFT level. We focus our theoretical discussion on the integration of this approach into the framework of Car–Parrinello AIMD, and highlight the central role played by the MLWF-product potential (i.e., the solution of Poisson’s equation for each corresponding MLWF-product density) in the evaluation of the EXX energy and wave function forces. We then provide a comprehensive description of the exx algorithm implemented in the open-source Quantum ESPRESSO program, which employs a hybrid MPI/OpenMP parallelization scheme to efficiently utilize the high-performance computing (HPC) resources available on current- and next-generation supercomputer architectures. Furthermore, this is followed by a critical assessment of the accuracy and parallel performance (e.g., strong and weak scaling) of this approach when AIMD simulations of liquid water are performed in the canonical (NVT) ensemble. With access to HPC resources, we demonstrate that exx enables hybrid DFT-based AIMD simulations of condensed-phase systems containing 500–1000 atoms (e.g., (H₂O)₂₅₆) with a wall time cost that is comparable to that of semilocal DFT. In doing so, exx takes us one step closer to routinely performing AIMD simulations of complex and large-scale condensed-phase systems for sufficiently long time scales at the hybrid DFT level of theory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Dynamics of Phospholipid Membranes beyond Thermal Undulations

Here, we investigated the molecular dynamics of unilamellar liposomes by neutron spin echo spectroscopy. We report the first experimental evidence of a short-range motion at the length scale of the size of the headgroup of a lipid. The associated mean squared displacement shows a t 0.26 dependence in the pico- to nanosecond region that indicates another process beyond the predictions of the Zilman–Granek (ZG) model (t 0.66 ) and translational diffusion (t 1 ). A comparison with theory shows that the observed low exponent is associated with a non-Gaussian transient trapping of lipid molecules in a local area and supports the continuous time random walk model. The analysis of the mean squared displacement leads to the important conclusion that the friction at the interface between water and liposomes plays a minor role. Center of mass diffusion of liposomes and transient trapping of lipids define the range in which the ZG model can be applied to analyze membrane fluctuations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Photoinduced Dynamics of Charge Separation: From Photosynthesis to Polymer–Fullerene Bulk Heterojunctions

Understanding charge separation and charge transport is crucial for improving the efficiency of organic solar cells. Their active media are based on organic molecules and polymers, serving as both light-absorbing and transport layers. The charge-transfer (CT) states play an important role, being intermediate for free carrier generation and charge recombination. Here, we use light-induced electron paramagnetic resonance spectroscopy to study the CT dynamics in blends of the polymers P3HT, PCDTBT, and PTB7 with the fullerene derivative C-60-PCBM. Time-resolved EPR measurements show strong spin-polarization patterns for all polymer-fullerene blends, confirming predominant generation of singlet CT states and partial orientation ordering near the donor-acceptor interface. These observations allow a comparison with charge separation processes in molecular donor-acceptor systems and in natural and artificial photosynthetic assemblies, and thus the elucidation of the initial steps of sequential CT in organic photovoltaic materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Structure, solvation, and dynamics of Mg{sup 2+}, Ca{sup 2+}, Sr{sup 2+}, and Ba{sup 2+} complexes with 3-hydroxyflavone and perchlorate anion in acetonitrile medium: A molecular dynamics simulation study

Molecular dynamics simulations of complexes of Mg{sup 2+}, Ca{sup 2+}, Sr{sup 2+}, and Ba{sup 2+} with 3-hydroxyflavone (flavonol, 3HF) and ClO {sub 4}{sup −} in acetonitrile were performed. The united atoms force field model was proposed for the 3HF molecule using the results of DFT quantum chemical calculations. 3HF was interpreted as a rigid molecule with two internal degrees of freedom, i.e., rotation of the phenyl ring and of the OH group with respect to the chromone moiety. The interatomic radial distribution functions showed that interaction of the cations with flavonol occurs via the carbonyl group of 3HF and it is accompanied with substitution of one of the acetonitrile molecules in the cations’ first solvation shells. Formation of the cation–3HF complexes does not have significant impact on the rotation of the phenyl ring with respect to the chromone moiety. However, the orientation of the flavonol's OH-group is more sensitive to the interaction with doubly charged cations. When complex with Mg{sup 2+} is formed, the OH-group turns out of the plane of the chromone moiety that leads to rupture of intramolecular H-bond in the ligand molecule. Complexation of Ca{sup 2+}, Sr{sup 2+}, and BaClO {sub 4}{sup +} with 3HF produces two structures with different OH-positions, as in the free flavonol with the intramolecular H-bond and as in the complex with Mg{sup 2+} with disrupted H-bonding. It was shown that additional stabilization of the [MgClO{sub 4}(3HF)]{sup +} and [BaClO{sub 4}(3HF)]{sup +} complexes is determined by strong affinity of perchlorate anion to interact with flavonol via intracomplex hydrogen bond between an oxygen atom of the anion and the hydrogen atom of the 3-hydroxyl group. Noticeable difference in the values of the self-diffusion coefficients for Kt{sup 2+} from one side and ClO {sub 4}{sup −}, 3HF, and AN in the cations’ coordination shell from another side implies quite weak interaction between cation, anion, and ligands in the investigated complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Vibrational dynamics of zero-field-splitting hamiltonian in gadolinium-based MRI contrast agents from ab initio molecular dynamics

The electronic relaxation of gadolinium complexes used as MRI contrast agents was studied theoretically by following the short time evolution of zero-field-splitting parameters. The statistical analysis of ab initio molecular dynamics trajectories provided a clear separation between static and transient contributions to the zero-field-splitting. For the latter, the correlation time was estimated at approximately 0.1 ps. The influence of the ligand was also probed by replacing one pendant arm of our reference macrocyclic complex by a bulkier phosphonate arm. In contrast to the transient contribution, the static zero-field-splitting was significantly influenced by this substitution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Dissociative chemisorption of methane on metal surfaces: Tests of dynamical assumptions using quantum models and ab initio molecular dynamics

The dissociative chemisorption of methane on metal surfaces is of great practical and fundamental importance. Not only is it the rate-limiting step in the steam reforming of natural gas, the reaction exhibits interesting mode-selective behavior and a strong dependence on the temperature of the metal. We present a quantum model for this reaction on Ni(100) and Ni(111) surfaces based on the reaction path Hamiltonian. The dissociative sticking probabilities computed using this model agree well with available experimental data with regard to variation with incident energy, substrate temperature, and the vibrational state of the incident molecule. We significantly expand the vibrational basis set relative to earlier studies, which allows reaction probabilities to be calculated for doubly excited initial vibrational states, though it does not lead to appreciable changes in the reaction probabilities for singly excited initial states. Sudden models used to treat the center of mass motion parallel to the surface are compared with results from ab initio molecular dynamics and found to be reasonable. Similar comparisons for molecular rotation suggest that our rotationally adiabatic model is incorrect, and that sudden behavior is closer to reality. Such a model is proposed and tested. A model for predicting mode-selective behavior is tested, with mixed results, though we find it is consistent with experimental studies of normal vs. total (kinetic) energy scaling. Models for energy transfer into lattice vibrations are also examined.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Nonlinear space charge dynamics in mixed ionic-electronic conductors: Resistive switching and ferroelectric-like hysteresis of electromechanical response

We performed self-consistent modelling of nonlinear electrotransport and electromechanical response of thin films of mixed ionic-electronic conductors (MIEC) allowing for steric effects of mobile charged defects (ions, protons, or vacancies), electron degeneration, and Vegard stresses. We establish correlations between the features of the nonlinear space-charge dynamics, current-voltage, and bending-voltage curves for different types of the film electrodes. A pronounced ferroelectric-like hysteresis of the bending-voltage loops and current maxima on the double hysteresis current-voltage loops appear for the electron-transport electrodes. The double hysteresis loop with pronounced humps indicates a memristor-type resistive switching. The switching occurs due to the strong nonlinear coupling between the electronic and ionic subsystems. A sharp meta-stable maximum of the electron density appears near one open electrode and moves to another one during the periodic change of applied voltage. Our results can explain the nonlinear nature and correlation of electrical and mechanical memory effects in thin MIEC films. The analytical expression proving that the electrically induced bending of MIEC films can be detected by interferometric methods is derived.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗