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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Ground-state properties of light 4 n self-conjugate nuclei in ab initio no-core Monte Carlo shell model calculations with nonlocal N N interactions

We report J π = 0 + ground-state energies and point-proton radii of 4 He, 8 Be, 12 C, 16 O, and 20 Ne nuclei calculated by the ab initio no-core Monte Carlo shell model with the JISP16 and Daejeon16 nonlocal NN interactions. Ground-state energies are obtained in the basis spaces up to seven oscillator shells (N shell =7) with several oscillator energies (ℏω) around the optimal oscillator energy for the convergence of ground-state energies. These energy eigenvalues are extrapolated to obtain estimates of converged ground-state energies in each basis space using energy variances of computed energy eigenvalues. We further extrapolate these energy-variance-extrapolated energies obtained in the finite basis spaces to infinite basis-space results with an empirical exponential form. This form features a dependence on the basis-space size but is independent of the value of ℏω used for the harmonic-oscillator basis functions. Point-proton radii for these states of atomic nuclei are also calculated following techniques employed for the energies. From these results, it is found that the Daejeon16 NN interaction provides good agreement with experimental data up to approximately 16 O, while the JISP16 NN interaction provides good agreement with experimental data up to approximately 12 C. Beyond these nuclei, the interactions produce overbinding accompanied by radii that are too small. These findings suggest and encourage further revisions of nonlocal NN interactions towards the investigation of nuclear structure in heavier-mass regions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Ab initio studies of 1,3,5,7-tetranitro-1,3,5,7-tetrazocine/1,3-dimethyl-2-imidazolidinone cocrystal under high pressure using dispersion corrected density functional theory

A detailed study of structural, electronic, and thermodynamic properties of 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)/1,3-dimethyl-2-imidazolidinone (DMI) cocrystal under the hydrostatic pressure of 0–100 GPa was performed by using dispersion-corrected density functional theory (DFT-D) method. The calculated crystal structure is in reasonable agreement with the experimental data at the ambient pressure. Based on the analysis of lattice constants, bond lengths, bond angles, and dihedral angles under compression, it is found that HMX molecules in HMX/DMI cocrystal are seriously distorted. In addition, as the pressure increases, the band gap decreases gradually, which suggests that HMX/DMI cocrystal is becoming more metallic. Some important intermolecular interactions between HMX and DMI are also observed in the density of states spectrum. Finally, its thermodynamic properties were characterized, and the results show that HMX/DMI cocrystal is more easily formed in the low pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Beyond MicroED: Ab Initio Structure Elucidation using 4D-STEM

Microcrystal electron diffraction (microED) has recently morphed into an increasingly mainstream technique in structural chemistry [1]. Its ability to interrogate nanocrystals orders of magnitude too small for conventional X-ray diffraction has enabled solid-state structure elucidation of several species previously considered impossible to solve using X-ray crystallography [2, 3]. Furthermore, selected area aperture-enabled microED remains thwarted by the presence of disordered, overlapping, or otherwise poorly diffracting domains, all of which routinely conspire to diminish data quality. Just as insufficient crystal size historically stymied conventional X-ray methods, these nanoscale defects frequently prohibit structure solution using classical microED.

Saha, Ambarneil↗
DOE OSTI2016

Full-dimensional quantum calculations of vibrational levels of NH 4 + and isotopomers on an accurate ab initio potential energy surface

Vibrational energy levels of the ammonium cation (NH 4 + ) and its deuterated isotopomers are calculated using a numerically exact kinetic energy operator on a recently developed nine-dimensional permutation invariant semiglobal potential energy surface fitted to a large number of high-level ab initio points. Like CH4, the vibrational levels of NH 4 + and ND 4 + exhibit a polyad structure, characterized by a collective quantum number P = 2(v 1 + v 3 ) + v 2 + v 4 . As a result, the low-lying vibrational levels of all isotopomers are assigned and the agreement with available experimental data is better than 1 cm –1 .

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

Full-dimensional diabatic potential energy surfaces including dissociation: The {sup 2}E{sup ″} state of NO{sub 3}

A scheme to produce accurate full-dimensional coupled diabatic potential energy surfaces including dissociative regions and suitable for dynamical calculations is proposed. The scheme is successfully applied to model the two-sheeted surface of the {sup 2}E{sup ″} state of the NO{sub 3} radical. An accurate potential energy surface for the NO{sub 3}{sup −} anion ground state is developed as well. Both surfaces are based on high-level ab initio calculations. The model consists of a diabatic potential matrix, which is expanded to higher order in terms of symmetry polynomials of symmetry coordinates. The choice of coordinates is key for the accuracy of the obtained potential energy surfaces and is discussed in detail. A second central aspect is the generation of reference data to fit the expansion coefficients of the model for which a stochastic approach is proposed. A third ingredient is a new and simple scheme to handle problematic regions of the potential energy surfaces, resulting from the massive undersampling by the reference data unavoidable for high-dimensional problems. The final analytical diabatic surfaces are used to compute the lowest vibrational levels of NO{sub 3}{sup −} and the photo-electron detachment spectrum of NO{sub 3}{sup −} leading to the neutral radical in the {sup 2}E{sup ″} state by full dimensional multi-surface wave-packet propagation for NO{sub 3} performed using the Multi-Configuration Time Dependent Hartree method. The achieved agreement of the simulations with available experimental data demonstrates the power of the proposed scheme and the high quality of the obtained potential energy surfaces.

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

Classical calculation of the equilibrium constants for true bound dimers using complete potential energy surface

The present paper aims at deriving classical expressions which permit calculation of the equilibrium constant for weakly interacting molecular pairs using a complete multidimensional potential energy surface. The latter is often available nowadays as a result of the more and more sophisticated and accurate ab initio calculations. The water dimer formation is considered as an example. It is shown that even in case of a rather strongly bound dimer the suggested expression permits obtaining quite reliable estimate for the equilibrium constant. The reliability of our obtained water dimer equilibrium constant is briefly discussed by comparison with the available data based on experimental observations, quantum calculations, and the use of RRHO approximation, provided the latter is restricted to formation of true bound states only.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Thermal transport in nanoporous holey silicon membranes investigated with optically induced transient thermal gratings

In this study, we use transient thermal gratings—a non-contact, laser-based thermal metrology technique with intrinsically high accuracy—to investigate room-temperature phonon-mediated thermal transport in two nanoporous holey silicon membranes with limiting dimensions of 120 nm and 250 nm, respectively. We compare the experimental results with ab initio calculations of phonon-mediated thermal transport according to the phonon Boltzmann transport equation (BTE) using two different computational techniques. We find that the calculations conducted within the Casimir framework, i.e., based on the BTE with the bulk phonon dispersion and diffuse scattering from surfaces, are in quantitative agreement with the experimental data and thus conclude that this framework is adequate for describing phonon-mediated thermal transport in silicon nanostructures with feature sizes of the order of 100 nm.

Duncan, Ryan A. (ORCID:0000000328318099)↗
DOE OSTI2019

Ab initio no-core Gamow shell-model calculations of multineutron systems

The existence of multineutron systems has always been a debatable question. Indeed, both internucleon correlations and a large continuum coupling occur in these states. We then employ the ab initio no-core Gamow shell model to calculate the resonant energies and widths of the trineutron and tetraneutron systems with realistic interactions. In this work, our results indicate that trineutron and tetraneutron are both unbound and bear broad widths. The calculated energy and width of tetraneutron are also comparable with recent experimental data. Moreover, our calculations suggest that the energy of trineutron is lower than that of tetraneutron, while its resonance width is also narrower. This strongly suggests that trineutron is more likely to be experimentally observed than tetraneutron. We thus suggest experimentalists to search for trineutron at low energy.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
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 OSTI2023

Correlation‐Driven Magnetic Frustration and Insulating Behavior of TiF 3

The halide perovskite TiF 3 , renowned for its intricate interplay between structure, electronic correlations, magnetism, and thermal expansion, is investigated. Despite its simple structure, understanding its low‐temperature magnetic behavior has been a challenge. Previous theories propose antiferromagnetic ordering. In contrast, experimental signatures for an ordered magnetic state are absent down to 10 K. The current study has successfully reevaluated the theoretical modeling of TiF 3 , unveiling the significance of strong electronic correlations as the key driver for its insulating behavior and magnetic frustration. In addition, frequency‐dependent optical reflectivity measurements exhibit clear signs of an insulating state. The analysis of the calculated magnetic data gives an antiferromagnetic exchange coupling with a net Weiss temperature of order 25 K as well as a magnetic response consistent with aS = 1/2 local moment per Ti 3+ . Yet, the system shows no susceptibility peak at this temperature scale and appears free of long‐range antiferromagnetic order down to 1 K. Extending ab initio modeling of the material to larger unit cells shows a tendency for relaxing into a noncollinear magnetic ordering, with a shallow energy landscape between several magnetic ground states, promoting the status of this simple, nearly cubic perovskite structured material as a candidate spin liquid.

Materials Science↗
DOE OSTI2020

Density Functionals for Hydrogen Storage: Defining the H2Bind275 Test Set with Ab Initio Benchmarks and Assessment of 55 Functionals

Efficient and high-capacity storage materials are indispensable for a hydrogen-based economy. In silico tools can accelerate the process of discovery of new adsorbent materials with optimal hydrogen adsorption enthalpies. Density functional theory is well-poised to become a very useful tool for enabling high-throughput screening of potential materials. In this work, we have identified density functional approximations that provide good performance for hydrogen binding applications following a two-pronged approach. First, we have compiled a data set (H2Bind275) that comprehensively represents the hydrogen binding problem capturing the chemical and mechanistic diversity in the binding sites encountered in hydrogen storage materials. We have also computed reference interaction energies for this data set using coupled-cluster theory. Second, we have assessed the performance of 55 density functional approximations for predicting H 2 interaction energies and have identified two hybrid density functionals (ωB97X-V and ωB97M-V), two double hybrid density functionals (DSD-PBEPBE-D3(BJ) and PBE0-DH), and one semilocal density functional (B97M-V) as the best performing ones. We have recommended the addition of empirical dispersion corrections to systematically underbinding density functionals such as revPBE, BLYP, and B3LYP for improvements in performance at negligible additional cost. We have also recommended the usage of the def2-TZVPP basis set as it represents a good compromise between accuracy and cost, limiting the finite basis set errors to less than 1 kJ/mol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Phase Equilibrium of Water with Hexagonal and Cubic Ice Using the SCAN Functional

Machine learning models are rapidly becoming widely used to simulate complex physicochemical phenomena with ab initio accuracy. Here, we use one such model as well as direct density functional theory (DFT) calculations to investigate the phase equilibrium of water, hexagonal ice (Ih), and cubic ice (Ic), with an eye toward studying ice nucleation. The machine learning model is based on deep neural networks and has been trained on DFT data obtained using the SCAN exchange and correlation functional. We use this model to drive enhanced sampling simulations aimed at calculating a number of complex properties that are out of reach of DFT-driven simulations and then employ an appropriate reweighting procedure to compute the corresponding properties for the SCAN functional. This approach allows us to calculate the melting temperature of both ice polymorphs, the driving force for nucleation, the heat of fusion, the densities at the melting temperature, the relative stability of ices Ih and Ic, and other properties. We find a correct qualitative prediction of all properties of interest. In some cases, quantitative agreement with experiment is better than for state-of-the-art semiempirical potentials for water. Furthermore, our results also show that SCAN correctly predicts that ice Ih is more stable than ice Ic.

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

Analysis of the structural, electronic and optic properties of Ni doped MgSiP{sub 2} semiconductor chalcopyrite compound

The structural, electronic band structure and optic properties of the Ni doped MgSiP{sub 2} chalcopyrite compound have been performed by using first-principles method in the density functional theory (DFT) as implemented in Vienna Ab-initio Simulation Package (VASP). The generalized gradient approximation (GGA) in the scheme of Perdew, Burke and Ernzerhof (PBE) is used for the exchange and correlation functional. The present lattice constant (a) follows generally the Vegard’s law. The electronic band structure, total and partial density of states (DOS and PDOS) are calculated. We present data for the frequency dependence of imaginary and real parts of dielectric functions of Ni doped MgSiP{sub 2}. For further investigation of the optical properties the reflectivity, refractive index, extinction coefficient and electron energy loss function are also predicted. Our obtained results indicate that the lattice constants, electronic band structure and optical properties of this compound are dependent on the substitution concentration of Ni.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Collisional excitation of interstellar PN by H 2 : New interaction potential and scattering calculations

Here, rotational excitation of interstellar PN molecules induced by collisions with H 2 is investigated. We present the first ab initio four-dimensional potential energy surface (PES) for the PN–H 2 van der Waals system. The PES was obtained using an explicitly correlated coupled cluster approach with single, double, and perturbative triple excitations [CCSD(T)-F12b]. The method of interpolating moving least squares was used to construct an analytical PES from these data. The equilibrium structure of the complex was found to be linear, with H 2 aligned at the N end of the PN molecule, at an intermolecular separation of 4.2 Å. The corresponding well-depth is 224.3 cm -1 . The dissociation energies were found to be 40.19 cm -1 and 75.05 cm -1 for complexes of PN with ortho-H 2 and para-H 2 , respectively. Integral cross sections for rotational excitation in PN–H 2 collisions were calculated using the new PES and were found to be strongly dependent on the rotational level of the H 2 molecule. These new collisional data will be crucial to improve the estimation of PN abundance in the interstellar medium from observational spectra.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1979

An ab initio investigation of the structure, vibrational frequencies, and intensities of HO2 and HOCl

The infrared spectral intensities for HOCl and HO2 have been calculated using a new ab initio technique. Theoretical results for the geometries, vibrational frequencies, and the dipole moments of these species are also reported. All of the calculations were performed at the SCF level using near Hartree-Fock quality basis sets. The results for the molecular geometries and the vibrational frequencies are in good agreement with available experimental data. It is believed that the computed intensities are accurate to at least 50%. The results should be helpful in attempts to determine the stratospheric abundance of HOCl and HO2 by in situ infrared spectroscopic measurements.

Komornicki, A.↗
DOE OSTI2023

Paramagnetic electronic structure of CrSBr: Comparison between ab initio G W theory and angle-resolved photoemission spectroscopy

Here, we explore the electronic structure of paramagnetic CrSBr by comparative first-principles calculations and angle-resolved photoemission spectroscopy. We theoretically approximate the paramagnetic phase using a supercell hosting spin configurations with broken long-range order and applying quasiparticle self-consistent GW theory, without and with the inclusion of excitonic vertex corrections to the screened Coulomb interaction (QS GW and QS $G\widehat{W}$, respectively). Comparing the quasiparticle band-structure calculations to angle-resolved photoemission data collected at 200 K results in excellent agreement. This allows us to qualitatively explain the significant broadening of some bands as arising from the broken magnetic long-range order and/or electronic dispersion perpendicular to the quasi-two-dimensional layers of the crystal structure. The experimental band gap at 200 K is found to be at least 1.51 eV at 200 K. At lower temperature, no photoemission data can be collected as a result of charging effects, pointing towards a significantly larger gap, which is consistent with the calculated band gap of approximately 2.1 eV.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Theoretical Study of Mechanical Stability and Physical Properties of Co{sub 2}V{sub 1−x}Zr{sub x}Ga

The mechanical stability and physical properties of Co{sub 2}V{sub 1−x}Zr{sub x}Ga (x = 0, 0.25, 0.50, 0.75, 1) have been predicted by using ab initio calculations based on density functional theory. The exchange-correlation potentials are treated within the generalized gradient approximation-Perdew-Burke and Ernzerhhof (GGA-PBE) and Hubbard coefficient (GGA+U). The investigated minimum lattice parameter of Co{sub 2}VGa is 5.8030 Å, which is in excellent agreement with available experimental and theoretical data and for Co{sub 2}V{sub 1−x}Zr{sub x}Ga (x = 0.25, 0.50, 0.75, 1) are 5.9026, 5.9625, 6.0060 and 6.0021 Å, respectively, which are predicted for the very first time. The total magnetic moments decrease with increasing concentration of x in Co{sub 2}V{sub 1−x}Zr{sub x}Ga, in agreement with the Slater-Pauling rule. Band structure and density of states calculations show that the minority spin channel exhibits band gaps of 0.43, 0.46, 0.52, 0.54 and 1.01 eV for GGA+U scheme around the Fermi level confirming that all the studied composites are half-metallic in nature. In this study, we also studied the elastic constant and it is established that all the materials are mechanically stable and ductile in nature. The Co{sub 2}VGa is stiffer than other materials and all the studied composites have anisotropic behaviour. Moreover, by using a quasi-harmonic Debye model and calculated elastic constant, the Debye temperature and temperature-dependent constant volume heat capacity have been investigated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗