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All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Deep-Learning Interatomic Potential Connects Molecular Structural Ordering to the Macroscale Properties of Polyacrylonitrile

Polyacrylonitrile (PAN) is an important commercial polymer, bearing atactic stereochemistry resulting from nonselective radical polymerization. As such, an accurate, fundamental understanding of governing interactions among PAN molecular units is indispensable for advancing the design principles of final products at reduced processability costs. While ab initio molecular dynamics (AIMD) simulations can provide the necessary accuracy for treating key interactions in polar polymers, such as dipole–dipole interactions and hydrogen bonding, and analyzing their influence on the molecular orientation, their implementation is limited to small molecules only. Herein, we show that the neural network interatomic potentials (NNIPs) that are trained on the small-scale AIMD data (acquired for oligomers) can be efficiently employed to examine the structures and properties at large scales (polymers). NNIP provides critical insight into intra- and interchain hydrogen-bonding and dipolar correlations and accurately predicts the amorphous bulk PAN structure validated by modeling the experimental X-ray structure factor. Furthermore, the NNIP-predicted PAN properties, such as density and elastic modulus, are in good agreement with their experimental values. Overall, the trend in the elastic modulus is found to correlate strongly with the PAN structural orientations encoded in the Hermans orientation factor. In conclusion, this study enables the ability to predict the structure–property relations for PAN and analogues with sustainable ab initio accuracy across scales.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Low-temperature thermal properties of Fe-doped Co 3 Sn 2 S 2

Recent studies on Co 3 Sn 2 S 2 usually focus on its electrical and magnetic behaviors as it is a Weyl semimetal, half metal, and anisotropic ferromagnet at the same time. Only a few works looked into its low-temperature thermal characteristics. Here, in this study, we report the low-temperature heat capacity, Seebeck coefficient, and thermal conductivity of Fe-doped Co 3 Sn 2 S 2 , as well as the formation energies, band structures, and spin-polarized density of states calculated by using the spin-polarized relativistic Korringa-Kohn-Rostoker – Green’s function method (sprKKR). The heat capacity data reveal that the sample with a heavier Fe-doping level has a lower Debye temperature and a larger density of states at the Fermi level, the latter confirmed by our ab initio computational results. The Fe-doping increases the absolute value of the Seebeck coefficient at temperatures above 120 K and dramatically suppresses the thermal conductivity. Furthermore, we discuss how the external applied magnetic field affects the Seebeck coefficient and thermal conductivity of pristine Co 3 Sn 2 S 2 . Our findings provide a detailed reference and may stimulate future investigations into the low-temperature thermal transport properties of Co 3 Sn 2 S 2 and other compounds in the shandite family.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ab initio informed 20 Ne(p, pα) 16 O reaction elucidates the emergence of alpha clustering from chiral potentials

We report on the first ab initio informed α knock-out reaction in the intermediate-mass region, with the aim to probe the underlying chiral potential and its impact on the emergence of alpha clustering in this mass region. The theoretical predictions of the α+ 16 O clustering in the 20 Ne ground state, based on the ab initio symmetry-adapted no-core shell model with continuum, yield a triple differential cross section for 20 Ne(p, pα) 16 O that is in a remarkable agreement with the data. This allows us to examine predictions of surface and in-medium α-cluster features that emerge from the underlying realistic nucleon-nucleon interaction with no parameters fitted to nuclear data beyond the two-body system, and to compare these to the successful antisymmetrized molecular dynamics approach.

Sargsyan, G. H. [Michigan State Univ., East Lansin↗
DOE OSTI2014

Full-dimensional characterization of photoelectron spectra of HOCO{sup −} and DOCO{sup −} and tunneling facilitated decay of HOCO prepared by anion photodetachment

The photodetachment of both the HOCO{sup −} and DOCO{sup −} anions is investigated using full-dimensional quantum wave packets on new ab initio based global potential energy surfaces for both the neutral and anionic species. The calculated electron affinities and neutral fundamental vibrational frequencies of both isotopomers are in good agreement with available experimental data. The measured photoelectron spectra are also accurately reproduced, further validating the accuracy of the potential energy surfaces. In addition, strong mode specificity is found in the lifetimes of the HOCO vibrational features and the tunneling facilitated predissociation rates to H + CO{sub 2} are rationalized using the recently proposed sudden vector projection model.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Transferable density functional tight binding for carbon, hydrogen, nitrogen, and oxygen: Application to shock compression

A new parameterization for density functional tight binding (DFTB) theory, lanl31, has been developed for molecules containing carbon, hydrogen, nitrogen, and oxygen. Optimal values for the Hubbard Us, on-site energies, and the radial dependences of the bond integrals and repulsive potentials were determined by numerical optimization using simulated annealing to a modest database of ab initio-calculated atomization energies and interatomic forces. The transferability of the optimized DFTB parameterization has been assessed using the CHNO subset of the QM-9 database [R. Ramakrishnan et al., Sci. Data, 1, 140022 (2014)]. These analyses showed that the errors in the atomization energies and interatomic forces predicted by our model are small and in the vicinity of the di erences between density functional theory calculations with di erent basis sets and exchange-correlation functionals. Good correlations between the molecular dipole moments and HOMO-LUMO gaps predicted by lanl31 and the QM-9 data set are also found. Furthermore, the errors in the atomization energies and forces derived from lanl31 are signi cantly smaller than those obtained from the ReaxFF-lg reactive force eld for organic materials [L. Liu et al., J. Phys. Chem. A, 115, 11016 (2011)]. The lanl31 DFTB parameterization for C, H, N, and O has been applied the to the molecular dynamics simulation of the principal Hugoniot of liquid nitromethane, liquid benzene, liquid nitrogen, pentaerythritol tetranitrate, trinitrotoluene, and cyclotetramethylene tetranitramine. The computed and measured Hugoniot loci are in excellent agreement with experiment and we discuss the sensitivity of the loci to the underestimated shock heating that is a characteristic of classical molecular dynamics simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Modeling the aqueous interface of amorphous TiO 2 using deep potential molecular dynamics

Amorphous titanium dioxide (a-TiO 2 ) is widely used as a coating material in applications such as electrochemistry and self-cleaning surfaces where its interface with water has a central role. However, little is known about the structures of the a-TiO 2 surface and aqueous interface, particularly at the microscopic level. In this work, we construct a model of the a-TiO 2 surface via a cut-melt-and-quench procedure based on molecular dynamics simulations with deep neural network potentials (DPs) trained on density functional theory data. After interfacing the a-TiO 2 surface with water, we investigate the structure and dynamics of the resulting system using a combination of DP-based molecular dynamics (DPMD) and ab initio molecular dynamics (AIMD) simulations. Both AIMD and DPMD simulations reveal that the distribution of water on the a-TiO 2 surface lacks distinct layers normally found at the aqueous interface of crystalline TiO 2 , leading to an ~10 times faster diffusion of water at the interface. Bridging hydroxyls (Ti 2 –O b H) resulting from water dissociation decay several times more slowly than terminal hydroxyls (Ti–O w H) due to fast Ti–O w H 2 → Ti–O w H proton exchange events. These results provide a basis for a detailed understanding of the properties of a-TiO 2 in electrochemical environments. Moreover, the procedure of generating the a-TiO 2 -interface employed here is generally applicable to studying the aqueous interfaces of amorphous metal oxides.

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

Superdiffusive heat conduction in semiconductor alloys. II. Truncated Lévy formalism for experimental analysis

Nearly all experimental observations of quasiballistic heat flow are interpreted using Fourier theory with modified thermal conductivity. Detailed Boltzmann transport equation (BTE) analysis, yet, reveals that the quasi-ballistic motion of thermal energy in semiconductor alloys is no longer Brownian but instead exhibits Lévy dynamics with fractal dimension α < 2 . In this work, we present a framework that enables full three-dimensional experimental analysis by retaining all essential physics of the quasiballistic BTE dynamics phenomenologically. A stochastic process with just two fitting parameters describes the transition from pure Lévy superdiffusion as short length and time scales to regular Fourier diffusion. The model provides accurate fits to time domain thermoreflectance raw experimental data over the full modulation frequency range without requiring any “effective” thermal parameters and without any a priori knowledge of microscopic phonon scattering mechanisms. Identified α values for InGaAs and SiGe match ab initio BTE predictions within a few percent. Our results provide experimental evidence of fractal Lévy heat conduction in semiconductor alloys. The formalism additionally indicates that the transient temperature inside the material differs significantly from Fourier theory and can lead to improved thermal characterization of nanoscale devices and material interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Optical Constants of an Apatite Single Crystal in the IR Range of 6–28 μm

On the basis of polarized IR-reflection spectra in the range of 5000–350 cm{sup –1} measured from the natural face of an optically transparent fluorapatite single crystal, components of a complex refractive index (optical constants) for radiation-vector orientations E ‖ c and E ⊥ c have been calculated by the Kramers–Kronig method. The fluorapatite single crystal has been chosen from several samples: it contains a minimum amount of impurities and has a high degree of crystallinity in accordance with the criteria of IR spectroscopy and Raman spectroscopy. Tabular data on optical constants for ordinary and extraordinary rays are given for the IR range of 6–28 μm. The obtained absorption spectra are compared with the results of quantum-chemical ab initio calculations within the B3LYP simulation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Atomic and Electronic Structure of 3C-SiC(111)-(2√3×2√3)-R30° Surface

The theoretical studies and ab initio calculations have been carried out for the first time for atomic and electronic structure of four variants of 3C-SiC(111)-(2√3×2√3)-R30° surface with Si- terminated surface: initial, relaxed, reconstructed, and relaxed after reconstruction. The surface was simulated in the layered superlattice approximation by a system of thin films (slabs) with a thickness of 12 atomic layers separated by vacuum gaps of ~16 Å. In order to close the dangling carbon bonds, the opposite side of the film was complemented with 12 atoms of hydrogen. Ab initio calculations were performed by means of QUANTUM ESPRESSO program based on the density functional theory. It is shown that the reconstruction makes the atomic layers split. Our previous works and experimental data show that these splits are observed at reconstructions of surface (111) in crystals with the sphalerite structure. The band structures of four alternative slabs are calculated and analyzed. Total and layer-by-layer valence electron state densities are calculated for four top layers. It is shown that the real surface has a metallic conductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Nucleon-Nucleon Scattering Up to N5LO in Chiral Effective Field Theory

During the past few decades a large effort has been made toward describing the NN interaction in the framework of chiral Effective Field Theory (EFT). The main idea is to exploit the symmetries of QCD to obtain an effective theory for low energy nuclear systems. In 2003, the first accurate charge-dependent NN potential in this scheme was developed and it has been applied to many ab-initio calculations, opening the possibility to study nuclear systems in a systematic and accurate way. It was shown that the fourth order (N3LO) was necessary and sufficient to describe the NN scattering data with a χ 2 /d.o.f on the order of so-called high precision potentials. However the systematics of chiral EFT also allow to relate two- and many-body interactions in a well-defined way. Since many-body forces make their first appearance at higher order, they are substantially smaller than their two-body counterparts, but may never-the-less be crucial for some processes. Thus, there are observables where they can have a big impact and, for example, there are indications that they solve the long standing Ay puzzle of N-d scattering. The last few years, have also seen substantial progress toward higher orders of chiral EFT which was motivated by the fact that only three-body forces of rather high order may solve some outstanding issues in microscopic nuclear structure and reactions. In this chapter we will review the latest contributions of the authors to development of chiral EFT based potentials up to N4LO as well as first calculations conducted for NN scattering at N5LO.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

The Electrochemical Sodiation of Sb Investigated by Operando X-ray Absorption and 121 Sb Mössbauer Spectroscopy: What Does One Really Learn?

In this study, we want to highlight the assets and restrictions of X-ray absorption spectroscopy (XAS) and Mössbauer spectroscopy for investigating the mechanism of the electrochemical reaction of antimony electrode materials vs. Na. For this, operando XAS was carried out during the first one and a half cycles, and the whole set of measured data was analysed using a statistical-chemometric approach, while low temperature Mössbauer spectroscopy measurements were carried out ex situ on selected samples stopped at different points of the electrochemical reaction. Complementary ab initio calculations were performed to support the experimental findings. Both techniques show that, upon the first sodiation, most Sb reacts with Na to form disordered Na 3 Sb. This step is accompanied by the formation of amorphous Sb as an intermediate. Upon inversion of the current Na 3 Sb is desodiated and an amorphous Sb phase, distinct from the pristine bulk Sb state, is gradually formed. However, both XAS and Mössbauer spectroscopy were unable to spot the formation of intermediate Na x Sb phases, which were evinced in previous works by operando Pair Distribution Function analyses. The results shown here clearly assign such failure to the intrinsic inability of both techniques to identify these intermediates.

36 MATERIALS SCIENCE↗
NASA NTRS2010

Can Vertical Profiles of Tropospheric Methane on Titan Be Derived from Radio-Occultation Soundings?

The intensity of the received signal at Earth in the radio occultations of Titan is attenuated both by refractive defocusing and pressure-induced absorption from N2-N2 and CH4-N2 pairs. Because the absorption strength is different for the two sets of pairs, matching the retrieved absorptivity profile can in principle yield the vertical variation in gaseous methane in the troposphere. There are two factors that make this difficult. The first is the propagation of noise in the phase and amplitude of the received signal in the absorption retrieval. The phase data is first inverted to retrieve vertical profiles of refractivity, from which the refractive defocusing is calculated. This is then subtracted from the observed. intensity attenuation of the received signal to generate a profile of atmospheric absorption. The second problem is the uncertainty in the pressure-induced absorption coefficients. Laboratory data at radio wavelengths is only available near room temperature (see, e.g., [1] for N2-N2), and the extrapolation to the low temperatures in Titan's troposphere is not well established. Ab initio calculations by Borysow et al. [2, 3] provide absorption coefficients at low temperatures and long wavelengths, but their accuracy has come into question. We present examples from Cassini radio occultations of Titan to illustrate the difficulties. For methane mole fractions in the lower troposphere comparable to that inferred from the Huygens probe (approximately 0.05), it will be difficult to separate the contributions of N2-N2 collisions from those of N2-CH4, collisions to the retrieved absorption. However, higher concentrations of CH4 and/or a higher signal-to-noise ratio from a future uplink experiment could result in a successful separation of the two components. However, key to this are highly accurate estimates of the absorption from a combination of laboratory measurements at love temperatures and long wavelengths, and possibly improved theoretical calculations.

Flasar, F. Michael↗
DOE OSTI2016

Optimization of an exchange-correlation density functional for water

We describe a method, that we call data projection onto parameter space (DPPS), to optimize an energy functional of the electron density, so that it reproduces a dataset of experimental magnitudes. Our scheme, based on Bayes theorem, constrains the optimized functional not to depart unphysically from existing ab initio functionals. The resulting functional maximizes the probability of being the “correct” parameterization of a given functional form, in the sense of Bayes theory. The application of DPPS to water sheds new light on why density functional theory has performed rather poorly for liquid water, on what improvements are needed, and on the intrinsic limitations of the generalized gradient approximation to electron exchange and correlation. Finally, we present tests of our water-optimized functional, that we call vdW-DF-w, showing that it performs very well for a variety of condensed water systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Atomistic Simulations for Thermophysical Properties of Uranium-Containing Halide Molten Salts

Characterizing the thermophysical properties in both fuel and coolant salts are critical in modeling, developing, process optimizing and utilizing molten salt reactors (MSRs), as these properties directly relate to operation metrics and can inform on the selection of candidate salts. The demand for consistent, accurate and publicly available thermophysical property data has become more apparent in recent years as interests have increased from molten salt reactor developers. There are a number of challenges in experimentally measuring properties such as thermal conductivity, viscosity, density and heat capacity , which have led to sparse and often times conflicting data points or molten salts in general. Additionally, there are a number of hazards to consider when synthesizing, storing, using, treating and disposing of molten salts. With the advances in computational capabilities over the last 10 years, the use of atomistic simulations can be implemented to support these efforts. The primary objective of this work is characterize the thermophysical transport properties in a number of molten chloride salts, and in particular NaCl-UCl 3 using ab-initio molecular dynamic (AIMD) simulations. In this binary salt the UCl 3 acts as the primary fissile material and NaCl acts as a carrier salt due with its’ high solubility for actinides A number of studies on the thermophysical properties of NaCl-UCl 3 have been published but there is not a vast amount of viscosity data for this system. In 1975, Desyatnik, et al published a study reporting dynamic viscosities that were calculated from kinematic viscosity measurements, and using the coefficients provided the viscosity in a 70:30 NaCl:UCl 3 mixture is 2.29 cP and 2.88 for a 60:40 mixture. Termini et al. recently reported viscosities in the range of 2.75 – 3 cP for the 63:37 NaCl-UCl 3 mixture in the same temperature range using rolling ball viscosity measurements. Computational viscosity of a similar mixture (64:36) can be obtained from the work Andersson et al. using the reported diffusion coefficients, and the hydrodynamic radius from the pair-radial distribution functions (RDFs). Using Eq (1) (vida infra), the viscosity would be 2.50 cP at 1100K. This is not to say that these values are incorrect due to the varying reported values, but aims to highlight the necessity of this work. The data reported in this ongoing work are computations on a 64:36 mixture of NaCl-UCl 3 at 987K. This work is likely to be expanded into varying concentrations of this mixture along with the inclusion of other salt candidate mixtures.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2015

Lattice stability and high-pressure melting mechanism of dense hydrogen up to 1.5 TPa

Lattice stability and metastability, as well as melting, are vital features of the physics and chemistry of dense hydrogen. Using ab initio molecular dynamics (AIMD), the classical superheating limit and melting line of metallic hydrogen are investigated up to 1.5 TPa. The computations show that the classical superheating degree is about 100 K, and the classical melting curve becomes flat at a level of 350 K when beyond 500 GPa. This data allows us to estimate the well depth and the potential barriers that must be overcome when the crystal melts. Inclusion of nuclear quantum effects (NQE) using path integral molecular dynamics (PIMD) predicts that both superheating limit and melting temperature are lowered to below room temperature, but the latter never reaches absolute zero. Detailed analysis indicates that the melting is thermally activated, rather than driven by pure zero-point motion (ZPM). This argument was further supported by extensive PIMD simulations, demonstrating the stability of Fddd structure against liquefaction at low temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Structural water in amorphous carbonate minerals: ab initio molecular dynamics simulations of X-ray pair distribution experiments

Water is known to play a controlling role in directing mineralization pathways and stabilizing metastable amorphous intermediates in hydrous carbonate mineral MCO 3 ·nH 2 O systems, where M 2+ is a divalent metal cation. Despite this recognition, the nature of the controls on crystallization is poorly understood, largely owing to the difficulty in characterizing the dynamically disordered structures of amorphous intermediates at the atomic scale. Here, we present a series of atomistic models, derived from ab initio molecular dynamics simulation, across a range of experimentally relevant cations (M = Ca, Mg, Sr) and hydration levels (0 ≤ n ≤ 2). Theoretical simulations of the dependence of the X-ray pair distribution function on the hydration level n show good agreement with available experimental data and thus provide further evidence for a lack of significant nanoscale structure in amorphous carbonates. Upon dehydration, the metal coordination number does not change significantly, but the relative extent of water dissociation increases, indicating that a thermodynamic driving force exists for water dissociation to accompany dehydration. Mg strongly favors monodentate conformation of carbonate ligands and shows a marked preference to exchange monodentate carbonate O for water O upon hydration, whereas Ca and Sr exchange mono- and bidentate carbonate ligands with comparable frequency. Finally, water forms an extensive hydrogen bond network among both water and carbonate groups that exhibits frequent proton transfers for all three cations considered suggesting that proton mobility is likely predominantly due to water dissociation and proton transfer reactions rather than molecular water diffusion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Pressure-dependent kinetics of cyclopentene consumption

Recent experiments have uncovered deficiencies in the ability of current kinetic models for cyclopentane combustion to predict species profiles as a function of O2 concentration. One likely source of these discrepancies flows from the limited availability of relevant kinetic data for key functionalized intermediates, requiring the use of distant analogies to estimate their rates. This work fills this gap for cyclopentene, the most prominent C5 intermediate in low-temperature cyclopentane combustion. Ab initio transition-state-theory based master equation methods are used to calculate pressure-dependent rate constants across four potential energy surfaces (C5H8 + OH, C5H8 + HO2, C5H7, and C5H7 + O2). Potential energy surfaces are characterized at the CCSD(T)-F12/cc-pVDZ-F12//revDSD-PBEP86-D3BJ/def2-TZVPP level of theory. Pressure-dependent rate constants and branching fractions are determined via 1D master equation calculations. The results are used to analyze the competition for radicals between cyclopentene and cyclopentane in the intermediate temperature region, and to identify the dominant pathways flowing from these initiation channels. On the OH initiation surface, we find good agreement with recently published experimental rate constants at high temperature, and we elucidate the complex temperature dependence below 800 K, where 𝜋𝜋 addition becomes prominent. On the HO2 initiation surface, we find an interesting alternative route to bicyclic ether formation via well-skipping over the HO2 adduct well. On the cyclopentenyl radical surface, the major product is cyclopentadiene plus H. On the cyclopentenyl + O2 surface, we find that resonance stabilization largely prevents the allylic radical from undergoing oxidation, whereas the alkylic radical has a somewhat elevated propensity for oxidation pathways relative to cyclopentyl.

ab initio calculations↗
DOE OSTI2015

Knockout beyond the dripline

In this contribution, we will describe neutron and proton removal from {sup 9}C and {sup 7}Be which are two particularly interesting nuclei entering the nucleo-synthesis pp-chain [1, 2]. Neutron and proton removal reactions have been used in the past twenty years to probe the single-particle structure of exotic nuclei. The core parallel-momentum distribution can give information on the angular momentum and spin of the nucleon initial state while the total removal cross section is sensitive to the asymptotic part of the initial wave function and also to the reaction mechanism. Because knockout is a peripheral reaction from which the Asymptotic Normalization Constant (ANC) of the single-particle wave function can be extracted, it has been used as an indirect method to obtain the rate of reactions like {sup 8}B(p,γ){sup 9}C or {sup 7}Be(p,γ){sup 8}B. Nucleon removal has recently been applied by the HiRA collaboration [3] to situations in which the remaining “core” is beyond the drip line, such as {sup 8}C and {sup 6}Be, unbound by one or more protons, and whose excitation-energy spectrum can be obtained by the invariant-mass method. By gating on the ground-state peak, “core” parallel-momentum distributions and total knockout cross sections have been obtained similar to previous studies with well-bound “cores”. In addition for each projectile, knock out to final bound states has also been obtained in several cases. We will report on the theoretical description and comparison to this experimental data for a few cases for which advances in the accuracy of the transfer-to-the continuum model [4, 5] have been made [6]. These include the use, when available, of “ab-initio” overlaps for the initial state [7] and in particular their ANC values [8]. Also, the construction of a nucleus-target folding potential for the treatment of the core-target S-matrix [9] using for the cores “ab-initio” densities [10] and state-of-the-art n−{sup 9}Be optical potentials [11]. Preliminary results and open problems will be discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗