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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

WFOT: A Wave Function Overlap Tool between Single- and Multi-Reference Electronic Structure Methods for Spectroscopy Simulation

We report the development of a novel diagnostic tool, named wave function overlap tool (WFOT), designed to evaluate the overlap between wave functions computed at single-reference [i.e., time-dependent density functional theory or configuration interaction singles (CIS)] and multireference (i.e., CASSCF/CASPT2) electronic structure levels of theory. It relies on truncating the single- and multireference WFs to CIS-like expansions spanning the same configurational space and maximizing the molecular orbital overlap by means of a unitary transformation. To demonstrate the functionality of the tool, we calculate the transient spectrum of acetylacetone by evaluating excited state absorption signals with multireference quality on top of single-reference on-the-fly dynamics simulations. Semiautomatic spectra generation is facilitated by interfacing the tool with the COBRAMM package, which also allows one to use WFOT with several quantum chemistry codes such as Gaussian, NWChem, and OpenMolcas. Finally, other exciting possibilities for the utilization of the code beyond the simulation of transient absorption spectroscopy are eventually discussed.

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

Energy spectrum of neutron-rich helium isotopes: Complex made simple

We demonstrate herein that the intricate energy spectrum of neutron-rich helium isotopes can be straightforwardly described by taking advantage of the low-energy properties of neutron-neutron interaction and the scale separation that is present in diluted drip-line systems. By using arguments based on the halo effective field theory, we further carry out a parameter reduction of the complex-energy configuration interaction framework in the s p d space, including resonant and scattering states. By constraining the core potential to α - n scattering phase shifts and adjusting the strength of the spin-singlet central neutron-neutron interaction, we reproduce experimental energies and widths of 5 – 8 He within tens of keV precision. We predict a parity inversion of narrow resonances in He 9 and show that the ground state of He 10 is an s -wave-dominated configuration that could decay through two-neutron emission. This threshold state can be viewed as a “double-halo” structure in an analogy to the atomic He 3 4 He 2 trimer.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

β decay of Si 38 , 40 ( T z = + 5 , + 6 ) to low-lying core excited states in odd-odd P 38 , 40 isotopes

Low-lying excited states in P 38,40 have been identified in the β decay of T z =+5,+6, Si 38,40 . Based on the allowed nature of the Gamow-Teller (GT) decay observed, these states are assigned spin and parity of 1 + and are core-excited 1p1h intruder states with a parity opposite to the ground state. The occurrence of intruder states at low energies highlights the importance of pairing and quadrupole correlation energies in lowering the intruder states despite the N=20 shell gap. Configuration interaction shell model calculations with the state-of-art SDPF-MU effective interaction were performed to understand the structure of these 1p1h states in the even-A phosphorus isotopes. States in P 40 with N=25 were found to have very complex configurations involving all the fp orbitals leading to deformed states as seen in neutron-rich nuclei with N≈28. The calculated GT matrix elements for the β decay highlight the dominance of the decay of the core neutrons rather than the valence neutrons.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1986

Theoretical study of the X 2Pi and A 2Sigma(+) states of CuO and CuS

Theoretical spectroscopic constants and dipole moments are determined for the X 2Pi and A 2Sigma(+) states of CuO and CuS, using extended Gaussian basis sets and incorporating correlation using both configuration interaction (CI) and coupled pair (CPF) methods. Relativistic corrections (Darwin plus mass velocity), included using first-order perturbation theory, are found to be relatively small. At the CPF level, significant configuration mixing with the 3d(9)4s(1) occupation occurs for both states, resulting in some bond shortening and a significant increase in the dissociation energy. The best spectroscopic parameters are in excellent agreement with experiment for both CuO and CuS. In analogy with CuO, CuS is predicted to have a yet unobserved 2Sigma(+) state near 10,000 per cm.

Langhoff, S. R.↗
NASA NTRS1996

Ab initio Potential-Energy Surfaces and Electron-Spin-Exchange Cross Sections for H-O2 Interactions

Accurate quartet- and doublet-state potential-energy surfaces for the interaction of a hydrogen atom and an oxygen molecule in their ground states have been determined from an ab initio calculation using large-basis sets and the internally contracted multireference configuration interaction method. These potential surfaces have been used to calculate the H-O2 electron-spin-exchange cross section; the square root of the cross section (in a(sub 0)), not taking into account inelastic effects, can be obtained approximately from the expressions 2.390E(sup -1/6) and 5.266-0.708 log10(E) at low and high collision energies E (in E(sub h)), respectively. These functional forms, as well as the oscillatory structure of the cross section found at high energies, are expected from the nature of the interaction energy. The mean cross section (the cross section averaged over a Maxwellian velocity distribution) agrees reasonably well with the results of measurements.

Stallcop, James R.↗
DOE OSTI2016

Magnetic Excitations and Electronic Interactions in Sr 2 CuTeO 6 : A Spin- 1 / 2 Square Lattice Heisenberg Antiferromagnet

Sr 2 CuTeO 6 presents an opportunity for exploring low-dimensional magnetism on a square lattice of S = 1/2 Cu 2+ ions. In this study, we employ ab initio multireference configuration interaction calculations to unravel the Cu 2+ electronic structure and to evaluate exchange interactions in Sr 2 CuTeO 6 . The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Lastly, using this methodology, which is quite general, we demonstrate that Sr 2 CuTeO 6 is an almost ideal realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Unusual behavior of Cooper minima of n s subshells in high- Z atoms

Here, a study of Cooper minima (CM) arising from the photoionization of 6$\textit{s}$, 5$\textit{s}$, and 4$\textit{s}$ subshells of high-$\textit{Z}$ atoms has been performed using Dirac-Fock (DF), two-channel relativistic-random-phase approximation (RRPA), and fully coupled RRPA. The results show huge splittings between $ns → εp_{3/2}$ and $ns → εp_{1/2}$ CM which increase with $\textit{Z}$ owing primarily to the relativistic interactions (spin orbit) that are attractive for the $εp_{1/2}$ final state but repulsive for the corresponding $εp_{3/2}$. In addition, it was found that correlation in the form of interchannel coupling (essentially configuration interaction in the final continuum states) plays a huge role in determining the location of the CM. For 6$\textit{s}$ photoionization, the 6$s → εp_{3/2}$ and 6$s → εp_{1/2}$ CM behave completely different as a function of $\textit{Z}$ . It was also found that for 5$\textit{s}$ and 4$\textit{s}$ photoionization, the CM move below the threshold, with increasing $\textit{Z}$ , and, at high enough $\textit{Z}$ , the 5s → εp3/2 and 4s → εp3/2 CM re-emerge into the continuum. The calculations have been carried out for the $\textit{ns}$ subshells of Hg ($\textit{Z}$ = 80), Rn ($\textit{Z}$ = 86), Ra ($\textit{Z}$ = 88), No ($\textit{Z}$ = 102), Cn ($\textit{Z}$ = 112), and Og ($\textit{Z}$ = 118).

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1985

System technology analysis of aeroassisted orbital transfer vehicles: Moderate lift/drag (0.75-1.5), Volume 1B, part 2, study results

A complete compilation of the results from Phase 2 of a study to identify and prioritize the technology payoffs of representative space based mid lift/drag ratio (L/D) aeroassisted orbit transfer vehicles (AOTV) and the cryofueled propulsion subsystem - configuration interactions is presented. Several combinations of basing and launch vehicle options, staging scenarios, missions (delivery, servicing, or manned round trip), and target orbits were considered. Space basing of an AOTV opens up numerous configuration opportunities. The size can exceed the launch vehicle cargo bay envelope by resorting to orbital assembly and the AOTV weight can exceed the launch vehicle capability. With the absence of fully fueled tanks during a ground based launch much lighter gossamer type structures are possible that may result in performance gains. At the Space Station, payload rearranging or manifesting may prove attractive. Several major conclusions regarding aerothermodynamics, aerodynamics, payload manifesting, the propulsion subsystem, and systems issues are discussed.

Source record↗
DOE OSTI2018

Origin of Panchromaticity in Multichromophore–Tetrapyrrole Arrays

Panchromatic absorbers that have robust photophysical properties enable new designs for molecular-based light-harvesting systems. Herein, we report experimental and theoretical studies of the spectral, redox, and excited-state properties of a series of perylene-monoimide–ethyne–porphyrin arrays wherein the number of perylene-monoimide units is stepped from one to four. In the arrays, a profound shift of absorption intensity from the strong violet-blue (B y and B x ) bands of typical porphyrins into the green, red and near-infrared (Q x and Q y ) regions stems from mixing of chromophore and tetrapyrrole molecular orbitals (MOs), which gives multiplets of MOs having electron density spread over the entire array. This reduces the extensive mixing between porphyrin excited-state configurations and the transition-dipole addition and subtraction that normally leads to intense B and weak Q bands. Reduced configurational mixing derives from moderate effects of the ethyne and perylene on the MO energies and a more substantial effect of electron-density delocalization to reduce the configuration-interaction energy. Quantitative oscillator-strength analysis shows that porphyrin intensity is also shifted into the perylene-like green-region absorption and that the ethyne linkers lend absorption intensity. The reduced porphyrin configurational mixing also endows the S 1 state with bacteriochlorin-like properties, including a 1-5 ns lifetime.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Diabatic Valence-Hole States in the C 2 Molecule: “Putting Humpty Dumpty Together Again”

Despite the long history of spectroscopic studies of the C$_2$ molecule, fundamental questions about its chemical bonding are still being hotly debated. The complex electronic structure of C$_2$ is a consequence of its dense manifold of near-degenerate, low-lying electronic states. A global multi-state diabatic model is proposed here to disentangle the numerous configuration interactions within four symmetry manifolds of C$_2$ ($^{1}\Pi_g$, $^{3}\Pi_g$, $^{1}\Sigma_u^+$, and $^{3}\Sigma_u^+$). The key concept of our model is the existence of two "valence-hole" configurations, $2\sigma_g^22\sigma_u^11\pi_{u}^33\sigma_g^2$ for $^{1,3}\Pi_g$ states and $2\sigma_g^22\sigma_u^11\pi_{u}^43\sigma_g^1$ for $^{1,3}\Sigma_u^+$ states that derive from $3\sigma_g\leftarrow2\sigma_u$ electron promotion. The lowest-energy state from each of the four C$_2$ symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear separation. Finally, as a result of their large binding energy (nominal bond order of 3) and correlation with the 2s$^2$2p$^2$+2s2p$^3$ separated-atom configurations, the presence of these valence-hole configurations has a profound impact on the $global$ electronic structure and unimolecular dynamics of C$_2$.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Modeling Strong Light-Matter Coupling in Correlated Systems: State-Averaged Cavity Quantum Electrodynamics Complete Active Space Self-Consistent Field Theory

The description of strongly correlated systems interacting with quantized cavity modes poses significant theoretical challenges due to the combinatorial scaling of electronic and photonic degrees of freedom. Recent advances addressing this complexity include cavity quantum electrodynamics (QED) generalizations of complete active space configuration interaction and density matrix renormalization group methods. In this work, we introduce a QED extension of state-averaged complete active space self-consistent field theory, which incorporates cavity-induced correlations through a second-order orbital optimization framework with robust convergence properties. The method is implemented using both photon number state and coherent state representations, with the latter showing robust origin invariance in the energies regardless of the completeness of the photonic Fock space. The implementation enables symmetry-free orbital relaxations to account for photon-mediated symmetry breaking in polaritonic systems. Numerical validation on lithium hydride, hydroxide anion, and magnesium hydride cation demonstrates that this method achieves significantly improved accuracy in modeling ground-state and polariton potential energy surfaces compared to QED-CASCI in a fixed orbital basis. In these studies, we reach sub-kcal/mol accuracy in potential energy surface in much smaller active spaces than are required for QED-CASCI. This advancement provides a more robust approach for studying cavity-altered chemical landscapes for ground and exited strongly coupled systems.

CASSCF↗
DOE OSTI2024

Barrier penetration in a discrete-basis formalism

The dynamics of a many-particle system are often modeled by mapping the Hamiltonian into a Schrödinger equation. An alternative approach is to solve the Hamiltonian equations directly in a model space of many-body configurations. In a previous paper the numerical convergence of the two approaches was compared with a simplified treatment of the Hamiltonian representation. Here, we extend the comparison to the nonorthogonal model spaces that would be obtained by the generator-coordinate method. With a suitable choice of the collective-variable grid, a configuration-interaction Hamiltonian can reproduce the Schrödinger dynamics very well. Furthermore, the method as implemented here requires that the barrier height is not much larger than the zero-point energy in the collective coordinates of the configurations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Shell-model coupled-cluster method for open-shell nuclei

An approach to derive effective shell-model interactions from microscopic nuclear forces is introduced here. The similarity transformed coupled-cluster Hamiltonian decouples the single-reference state of a closed-shell nucleus and provides us with a core for the shell model. We use a second similarity transformation to decouple a shell-model space from the excluded space. We show that the three-body terms induced by both similarity transformations are crucial for an accurate computation of ground and excited states. As a proof of principle we use a nucleon-nucleon interaction from chiral effective field theory, employ a 4 He core, and compute low-lying states of 6–8 He and 6–8 Li in p -shell model spaces. Our conclusions agree with benchmarks from full configuration interaction.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Efficient Implementation of NOCI-MP2 Using the Resolution of the Identity Approximation with Application to Charged Dimers and Long C–C Bonds in Ethane Derivatives

An efficient implementation of the perturb-then-diagonalize nonorthogonal configuration interaction method with second-order Møller-Plesset perturbation theory (NOCI-MP2) is introduced. Relative to other low scaling multireference perturbation theories, NOCI-MP2 often requires a much smaller active space because of the use of nonorthogonal reference configurations. Reworking the NOCI-MP2 equations with the resolution of the identity (RI) approximation enables the method to have the same memory requirements and computational scaling as single reference RI-MP2. The working equations are extended to include single substitutions as required when the reference determinants do not satisfy the Hartree-Fock equations. A detailed computational algorithm is presented along with timings to establish the performance of the implementation. NOCI-MP2 is applied to the binding energy and charge resonance energy in dication and monocation π dimers, as well as didiamantane ethane, and hexaphenylethane. A well-defined set of nonorthogonal determinants are obtained using absolutely localized molecular orbitals (ALMOs), as solutions to the self-consistent field for molecular interactions (SCF-MI) equations corresponding to covalent and ionic determinants. Agreement with experimental information where available, and other multireference methods, is satisfactory, with the use of an 0.3 au level shift to guard against large MP2 amplitudes. For didiamantane ethane and hexaphenylethane, large dispersion forces help stabilize the molecules despite the steric repulsion. However, in the case of hexaphenylethane, the energy penalty from the geometric distortion of the fragments critically weakens the bond.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Quantitative Evidence for Lanthanide-Oxygen Orbital Mixing in CeO 2 , PrO 2 , and TbO 2

We report that understanding the nature of covalent (band-like) vs ionic (atomic-like) electrons in metal oxides continues to be at the forefront of research in the physical sciences. In particular, the development of a coherent and quantitative model of bonding and electronic structure for the lanthanide dioxides, LnO 2 (Ln = Ce, Pr, and Tb), has remained a considerable challenge for both experiment and theory. Herein, relative changes in mixing between the O 2p orbitals and the Ln 4f and 5d orbitals in LnO 2 are evaluated quantitatively using O K-edge X-ray absorption spectroscopy (XAS) obtained with a scanning transmission X-ray microscope and density functional theory (DFT) calculations. For each LnO 2 , the results reveal significant amounts of Ln 5d and O 2p mixing in the orbitals of t 2g (σ-bonding) and e g (π-bonding) symmetry. The remarkable agreement between experiment and theory also shows that significant mixing with the O 2p orbitals occurs in a band derived from the 4f orbitals of a 2u symmetry (σ-bonding) for each compound. However, a large increase in orbital mixing is observed for PrO 2 that is ascribed to a unique interaction derived from the 4f orbitals of t 1u symmetry (σ- and π-bonding). Lastly, O K-edge XAS and DFT results are compared with complementary L 3 -edge and M 5,4 -edge XAS measurements and configuration interaction calculations, which shows that each spectroscopic approach provides evidence for ground state O 2p and Ln 4f orbital mixing despite inducing very different core-hole potentials in the final state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Accelerating eigenvalue computation for nuclear structure calculations via perturbative corrections

Subspace projection methods utilizing perturbative corrections have been proposed for computing the lowest few eigenvalues and corresponding eigenvectors of large Hamiltonian matrices. In this paper, we build upon these methods and introduce the term Subspace Projection with Perturbative Corrections (SPPC) method to refer to this approach. We tailor the SPPC for nuclear many-body Hamiltonians represented in a truncated configuration interaction subspace, i.e., the no-core shell model (NCSM). We use the hierarchical structure of the NCSM Hamiltonian to partition the Hamiltonian as the sum of two matrices. The first matrix corresponds to the Hamiltonian represented in a small configuration space, whereas the second is viewed as the perturbation to the first matrix. Eigenvalues and eigenvectors of the first matrix can be computed efficiently. Because of the split, perturbative corrections to the eigenvectors of the first matrix can be obtained efficiently from the solutions of a sequence of linear systems of equations defined in the small configuration space. These correction vectors can be combined with the approximate eigenvectors of the first matrix to construct a subspace from which more accurate approximations of the desired eigenpairs can be obtained. We show by numerical examples that the SPPC method can be more efficient than conventional iterative methods for solving large-scale eigenvalue problems such as the Lanczos, block Lanczos and the locally optimal block preconditioned conjugate gradient (LOBPCG) method. The method can also be combined with other methods to avoid convergence stagnation.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Evaluation of the excitation spectra with diffusion Monte Carlo on an auxiliary bosonic ground state

We aim to improve upon the variational Monte Carlo (VMC) approach for excitations replacing the Jastrow factor by an auxiliary bosonic (AB) ground state and multiplying it by a fermionic component factor. The instantaneous change in imaginary time of an arbitrary excitation in the original interacting fermionic system is obtained by measuring observables via the ground-state distribution of walkers of an AB system that is subject to an auxiliary effective potential. The effective potential is used to (i) drive the AB system’s ground-state configuration space toward the configuration space of the excitations of the original fermionic system and (ii) subtract from a diffusion Monte Carlo (DMC) calculation contributions that can be included in conventional approximations, such as mean-field and configuration interaction (CI) methods. In this novel approach, the AB ground state is treated statistically in DMC, whereas the fermionic component of the original system is expanded in a basis. The excitation energies of the fermionic eigenstates are obtained by sampling a fermion–boson coupling term on the AB ground state. We show that this approach can take advantage of and correct for approximate eigenstates obtained via mean-field calculations or truncated interactions. We demonstrate that the AB ground-state factor incorporates the correlations missed by standard Jastrow factors, further reducing basis truncation errors. Relevant parts of the theory have been tested in soluble model systems and exhibit excellent agreement with exact analytical data and CI and VMC approaches. In particular, for limited basis set expansions and sufficient statistics, AB approaches outperform CI and VMC in terms of basis size for the same systems. The implementation of this method in current codes, despite being demanding, will be facilitated by reusing procedures already developed for calculating ground-state properties with DMC and excitations with VMC.

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

Origin of Ferromagnetic Exchange Coupling in Donor–Acceptor Biradical Analogues of Charge-Separated Excited States

A new donor–acceptor biradical complex, Tp Cum,Me Zn(SQ-VD) (Tp Cum,Me Zn + = zinc(II) hydro-tris(3-cumenyl-5-methylpyrazolyl)borate complex cation; SQ = orthosemiquinone; VD = oxoverdazyl), which is a ground-state analogue of a charge-separated excited state, has been synthesized and structurally characterized. Here, the magnetic exchange interaction between the S = 1/2 SQ and the S = 1/2 VD within the SQ-VD biradical ligand is observed to be ferromagnetic, with $J_{SQ-VD}$ = +77 cm –1 (H = -2$J_{SQ-VD}\bar{S}_{SQ}·\bar{S}_{VD}$) determined from an analysis of the variable-temperature magnetic susceptibility data. The pairwise biradical exchange interaction in Tp Cum,Me Zn(SQ-VD) can be compared with that of the related donor–acceptor biradical complex Tp Cum,Me Zn(SQ-NN) (NN = nitronyl nitroxide, S = 1/2), where $J_{SQ-NN}$ ≅ +550 cm –1 . This represents a dramatic reduction in the biradical exchange by a factor of ~7, despite the isolobal nature of the VD and NN acceptor radical SOMOs. Computations assessing the magnitude of the exchange were performed using a broken-symmetry density functional theory (DFT) approach. These computations are in good agreement with those computed at the CASSCF NEVPT2 level, which also reveals an S = 1 triplet ground state as observed in the magnetic susceptibility measurements. A combination of electronic absorption spectroscopy and CASSCF computations has been used to elucidate the electronic origin of the large difference in the magnitude of the biradical exchange coupling between Tp Cum,Me Zn(SQ-VD) and Tp Cum,Me Zn(SQ-NN). A Valence Bond Configuration Interaction (VBCI) model was previously employed to highlight the importance of mixing an SQ SOMO → NNLUMO charge transfer configuration into the electronic ground state to facilitate the stabilization of the high-spin triplet (S = 1) ground state in Tp Cum,Me Zn(SQ-NN). Here, CASSCF computations confirm the importance of mixing the pendant radical (e.g., VD, NN) LUMO (VD LUMO and NN LUMO ) with the SOMO of the SQ radical (SQ SOMO ) for stabilizing the triplet, in addition to spin polarization and charge transfer contributions to the exchange. An important electronic structure difference between Tp Cum,Me Zn(SQ-VD) and Tp Cum,Me Zn(SQ-NN), which leads to their different exchange couplings, is the reduced admixture of excited states that promote ferromagnetic exchange into the Tp Cum,Me Zn(SQ-VD) ground state, and the intrinsically weaker mixing between the VD LUMO and the SQ SOMO compared to that observed for Tp Cum,Me Zn(SQ-NN), where this orbital mixing is significant. The results of this comparative study contribute to a greater understanding of biradical exchange interactions, which are important to our understanding of excited-state singlet–triplet energy gaps, electron delocalization, and the generation of electron spin polarization in both the ground and excited states of (bpy)Pt(CAT-radical) complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗