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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Covalency of Trivalent Actinide Ions with Different Donor Ligands: Do Density Functional and Multiconfigurational Wavefunction Calculations Corroborate the Observed “Breaks”?

A comprehensive ab initio study of periodic actinide–ligand bonding trends for trivalent actinides is performed. Relativistic density functional theory (DFT) and complete active-space (CAS) self-consistent field wavefunction calculations are used to dissect the chemical bonding in the [AnCl 6 ] 3– , [An(CN) 6 ] 3– , [An(NCS) 6 ] 3– , [An(S 2 PMe 2 ) 3 ], [An(DPA) 3 ] 3– , and [An(HOPO)] – series of actinide (An = U–Es) complexes. Except for some differences for the early actinide complexes with DPA, bond orders and excess 5f-shell populations from donation bonding show qualitatively similar trends in 5f n active-space CAS vs DFT calculations. The influence of spin–orbit coupling on donation bonding is small for the tested systems. Along the actinide series, chemically soft vs chemically harder ligands exhibit clear differences in bonding trends. There are pronounced changes in the 5f populations when moving from Pu to Am or Cm, which correlate with previously noted “breaks” in chemical trends. As a result, bonding involving 5f becomes very weak beyond Cm/Bk. We propose that Cm(III) is a borderline case among the trivalent actinides that can be meaningfully considered to be involved in ground-state 5f covalent bonding.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Defect energetics of concentrated solid-solution alloys from ab initio calculations: Ni 0.5 Co 0.5 , Ni 0.5 Fe 0.5 , Ni 0.8 Fe 0.2 and Ni 0.8 Cr 0.2

It has been shown that concentrated solid solution alloys possess unusual electronic, magnetic, transport, mechanical and radiation-resistant properties that are directly related to underlying chemical complexity. Because every atom experiences a different local atomic environment, the formation and migration energies of vacancies and interstitials in these alloys exhibit a distribution, rather than a single value as in a pure metal or dilute alloy. In this study, using ab initio calculations based on density functional theory and special quasirandom structure, we have characterized the distribution of defect formation energy and migration barrier in four Ni-based solid-solution alloys: Ni 0.5 Co 0.5 , Ni 0.5 Fe 0.5 , Ni 0.8 Fe 0.2 and Ni 0.8 Cr 0.2 . As defect formation energies in finite-size models depend sensitively on the elemental chemical potential, we have developed a computationally efficient method for determining it which takes into account the global composition and the local short-range order. In addition we have compared the results of our ab initio calculations to those obtained from available embedded atom method (EAM) potentials. Our results indicate that the defect formation and migration energies are closely related to the specific atomic size in the structure, which further determines the elemental diffusion properties. In conclusion, different EAM potentials yield different features of defect energetics in concentrated alloys, pointing to the need for additional potential development efforts in order to allow spatial and temporal scale-up of defect and simulations, beyond those accessible to ab initio methods.

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

Calculations on 1,8-naphthoquinone predict that the ground state of this diradical is a singlet: Calculations on 1,8-Naphthoquinone Predict that the Ground State of this Diradical Is a Singlet

(12/12)CASPT2, (16/14)CASPT2, B3LYP, and CCSD(T) calculations have been carried out on 1,8-Naphthoquinone (1,8-NQ), in order to predict the low-lying electronic states and their relative energies in this non-Kekulé quinone diradical. CASPT2 predicts a 1A1 ground state, with three other electronic states – 3B2, 3B1, and 1B1 – within about 10 kcal/mol of the ground state in energy. In contrast, CCSD(T) finds 3B2 to be the ground state, with 1A1 slightly higher in energy. Based on the comparisons of the singlet-triplet energy differences (?EST), calculated by CASPT2 and by CCSD(T) for two other non-Kekulé quinone diradicals, with the experimental values measured, by negative ion photoelectron spectroscopy (NIPES), it is predicted that NIPES experiments on 1,8-NQ•– will find that 1,8-NQ is a diradical with a singlet ground state.

Hrovat, David A.↗
DOE OSTI2015

Chemical expansion affected oxygen vacancy stability in different oxide structures from first principles calculations

We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. These results could serve as guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2019

Impact of solutes on the lattice parameters and elastic stiffness coefficients of hcp Fe from first-principles calculations

The hexagonal close-packed (hcp) -martensite phase in steels nucleates from the -austenite parent phase and can undergo further transformation to the -martensite phase or exist as a metastable phase depending on temperature, mechanical loading, and alloy chemistry. The solute-dependent lattice parameters and elastic stiffness coefficients of hcp Fe influence the mechanical properties of steels containing the -martensite phase, as well as the martensitic transformations between the phases. We use density functional theory to calculate the lattice parameters and of single-crystal hcp Fe as functions of solute concentration in the dilute limit for the substitutional solutes Al, B, Cu, Mn, and Si, and the octahedral interstitial solutes C and N. Our computationally efficient methodology separates the solute dependence of the into lattice strain and chemical bonding contributions. The computed data can be used to estimate the effect of solutes on polycrystalline elastic moduli and the strain energy associated with martensitic transformations. The data can also serve as inputs to microstructure-based models of multiphase steels containing the -martensite phase.

Fellinger, Michael R.↗
DOE OSTI2018

Accelerating nuclear configuration interaction calculations through a preconditioned block iterative eigensolver

We describe a number of recently developed techniques for improving the performance of large-scalenuclear configuration interaction calculations on high performance parallel computers. We show thebenefit of using a preconditioned block iterative method to replace the Lanczos algorithm that hastraditionally been used to perform this type of computation. The rapid convergence of the block iterativemethod is achieved by a proper choice of starting guesses of the eigenvectors and the construction of aneffective preconditioner. These acceleration techniques take advantage of special structure of the nuclearconfiguration interaction problem which we discuss in detail. The use of a block method also allows usto improve the concurrency of the computation, and take advantage of the memory hierarchy of modernmicroprocessors to increase the arithmetic intensity of the computation relative to data movement. Wealso discuss the implementation details that are critical to achieving high performance on massivelyparallel multi-core supercomputers, and demonstrate that the new block iterative solver is two to threetimes faster than the Lanczos based algorithm for problems of moderate sizes on a Cray XC30 system.

Shao, Meiyue↗
DOE OSTI2018

Kinetic-j : A computational kernel for solving the linearized Vlasov equation applied to calculations of the kinetic, configuration space plasma current for time harmonic wave electric fields

We present the Kinetic-j code, a computational kernel for evaluating the linearized Vlasov equation withapplication to calculating the kinetic plasma response (current) to an applied time harmonic wave electricfield. This code addresses the need for a configuration space evaluation of the plasma current to enablekinetic full-wave solvers for waves in hot plasmas to move beyond the limitations of the traditional Fourierspectral methods. We benchmark the kernel via comparison with the standard k-space forms of the hotplasma conductivity tensor.

Green, David L.↗
DOE OSTI2021

Characterization of Dosimetric Differences in Strut-Adjusted Volume Implant Treatment Plans Calculated With TG-43 Formalism and a Model-Based Dose Calculation Algorithm

To comprehensively characterize dosimetric differences between calculations with a commercial model-based dose calculation algorithm (MBDCA) and the TG-43 formalism in application to accelerated partial breast irradiation (APBI) with the strut-adjusted volume implant (SAVI) applicator.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2016

Hot zero power reactor calculations using the Insilico code

In this paper we describe the reactor physics simulation capabilities of the Insilico code. A description of the various capabilities of the code is provided, including detailed discussion of the geometry, meshing, cross section processing, and neutron transport options. Numerical results demonstrate that Insilico using an SP{sub N} solver with pin-homogenized cross section generation is capable of delivering highly accurate full-core simulation of various pressurized water reactor problems. Comparison to both Monte Carlo calculations and measured plant data is provided.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Monte Carlo calculation of large and small-angle electron scattering in air

A Monte Carlo method for angle scattering of electrons in air that accommodates the small-angle multiple scattering and larger-angle single scattering limits is introduced. The algorithm is designed for use in a particle-in-cell simulation of electron transport and electromagnetic wave effects in air. The method is illustrated in example calculations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Calculations of the pH-Dependent Onset Potential for CO Electrooxidation on Au(111)

CO electrooxidation on the Au(111) surface exhibits an onset potential that depends strongly on the pH of the electrolyte. In alkaline solution, the onset potential for CO electrooxidation is found, experimentally, to be 0.5 V lower than in acidic media on the reversible hydrogen electrode scale. This phenomenon is explained here with density functional theory which is used to calculate adsorbate binding energies including the electric double layer of the Au(111)/aqueous interface. Our model consists of a charged Au(111) slab and implicit solvation for the electrolyte. The double reference method is used to determine the potential-dependent CO electrooxidation reaction energetics. A microkinetic model, based upon the calculated reaction energetics, confi rms the lower onset potential in alkaline media. Our results show that there are three factors contributing to the lower onset potential in base: (1) stronger CO adsorption, (2) attraction between adsorbed CO and OH - , and (3) the high concentration of OH - in base.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗