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All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

The ezSpectra suite: An easy‐to‐use toolkit for spectroscopy modeling

Abstract A molecule's spectrum encodes information about its structure and electronic properties. It is a unique fingerprint that can serve as a molecular ID. Quantum chemistry calculations provide key ingredients for interpreting spectra, but modeling the spectra rarely ends there; it requires additional steps that entail combined treatments of electronic and nuclear degrees of freedom and account for specifics of the experimental setup (light energy, polarization, averaging over molecular orientations, temperature, etc.). This Software Focus article describes the ezSpectra suite, which currently comprises two stand‐alone open‐source codes: ezFCF and ezDyson . ezFCF calculates Franck–Condon factors, which yield vibrational progressions for polyatomic molecules, within the double‐harmonic approximation. ezDyson calculates absolute cross‐sections for photodetachment/photoionization processes and photoelectron angular distributions using Dyson orbitals computed by a quantum chemistry program. This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Theoretical and Physical Chemistry > Spectroscopy Software > Simulation Methods

Gozem, Samer↗
DOE OSTI2023

$\mathrm{DRAGON}$: A multi-GPU orbital-free density functional theory molecular dynamics simulation package for modeling of warm dense matter

As progress in electronic structure theoretical methods is made, ab initio molecular dynamics (MD) based on orbital-free density functional theory (OF-DFT) is becoming increasingly more successful at substituting the traditional, very accurate but computationally costly Kohn–Sham (KS) approach for simulations of matter at the challenging warm dense matter (WDM) regime. However, despite the significant cost alleviation of eliminating the dependence on the KS orbitals, OF-DFT MD runs require ~10 2 to 10 3 CPU cores running for days, or even weeks, for simulations of systems comprised of 10 2 to 10 3 atoms, depending on thermodynamic conditions. Here we present DRAGON, a multi-GPU OF-DFT MD code for fast and efficient simulations of WDM. With a relatively small allocation of resources (4 to 8 GPU devices) it can provide an order of magnitude speedup for simulations containing $\mathscr{O}$(10 4 ) atoms and target systems composed of $\mathscr{O}$(10 5 ) atoms at conditions within the WDM regime, which is currently outside the capabilities of CPU codes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Crystal structure and ion conductivity of a new mixed-anion phosphate LiMg{sub 3}(PO{sub 4})P{sub 2}O{sub 7}

A new lithium-containing phosphate, LiMg{sub 3}(PO{sub 4})P{sub 2}O{sub 7}, was prepared by a solid-state reaction, and it was characterized by an ab initio structure determination method on the basis of synchrotron powder X-ray diffraction data. LiMg{sub 3}(PO{sub 4})P{sub 2}O{sub 7} was found to be orthorhombic (space group Pnma) with lattice parameters a=9.0387(1) Å, b=10.6072(1) Å, c=8.3065(1) Å, and V=796.39(1) Å{sup 3}. The structure features infinite [Mg{sub 3}O{sub 10}]{sub ∞} layers that are parallel to the bc plane and that are interconnected along the a axis by PO{sub 4} and P{sub 2}O{sub 7} groups. The [Mg{sub 3}O{sub 10}]{sub ∞} layer contains Mg{sub 3}O{sub 14} trimers that are formed by three edge-shared MgO{sub 6} octahedra. The PO{sub 4} and P{sub 2}O{sub 7} groups are located alternatively between [Mg{sub 3}O{sub 10}]{sub ∞} layers. This gives rise to a three-dimensional framework that contains large tunnels along the directions [1 0 0] and [0 1 0]; the Li{sup +} ions are stabilized in these tunnels. AC impedance spectroscopy shows that LiMg{sub 3}(PO{sub 4})P{sub 2}O{sub 7} has an ionic conductivity of 3.40×10{sup −5} S cm{sup −1} at 769 K, with an activation energy of 1.17 eV. - Graphical abstract: Polyhedral view of LiMg3(PO4)P2O7. Li+ ions are represented by orange spheres, MgO6 groups by octahedra, and PO4 groups by tetrahedra. - Highlights: • New compound LiMg{sub 3}(PO{sub 4})P{sub 2}O{sub 7} is reported. • The crystal structure is investigated by synchrotron XRD analysis. • The structure features [Mg{sub 3}O{sub 10}]{sub ∞} layers with interconnecting PO{sub 4} and P{sub 2}O{sub 7} groups. • Correlation between the crystal structure and ionic conductivity is discussed.

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

High-Pressure Studies of Cesium Uranyl Chloride

Here, the pressure behavior of crystalline Cs 2 UO 2 Cl 4 has been explored using a diamond anvil cell. The uranyl fluorescence intensity decreases dramatically with increasing pressure. Using the O–U–O symmetric stretching frequency, an apparent linear decrease in bond length with increasing pressure was observed. A linear decrease in fluorescence intensity with increasing pressure was attributed to a large growth in the nonradiative relaxation, likely attributed to increased relaxation through phonon modes. Quantum theory of atoms in molecules calculations and ab initio wave function methods (CASSCF) support the U≡O bond in UO 2 2+ being highly sensitive to the bond distance, but negligibly affected by the U–Cl bond length.

anions↗
DOE OSTI2024

The Role of Spin–Orbit Coupling in the Linear Absorption Spectrum and Intersystem Crossing Rate Coefficients of Ruthenium Polypyridyl Dyes

The successful use of molecular dyes for solar energy conversion requires efficient charge injection, which in turn requires the formation of states with sufficiently long lifetimes (e.g., triplets). The molecular structure elements that confer this property can be found empirically, however computational predictions using ab initio electronic structure methods are invaluable to identify structure-property relations for dye sensitizers. The primary challenge for simulations to elucidate the electronic and nuclear origins of these properties is a spin-orbit interaction which drives transitions between electronic states. Here, in this work, we present a computational analysis of the spin-orbit corrected linear absorption cross sections and intersystem crossing rate coefficients for a derivative set of phosphonated tris(2,2'-bipyridine)ruthenium(2+) dye molecules. After sampling the ground state vibrational distributions, the predicted linear absorption cross sections indicate that the mixture between singlet and triplet states plays a crucial role in defining the line shape of the metal-to-ligand charge transfer bands in these derivatives. Additionally, an analysis of the intersystem crossing rate coefficients suggests that transitions from the singlet into the triplet manifolds are ultrafast with rate coefficients on the order of 10 13 s -1 for each dye molecule.

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

Ab Initio Kinetics of Hydrogen Abstraction from Methyl Acetate by Hydrogen, Methyl, Oxygen, Hydroxyl, and Hydroperoxy Radicals

The kinetics of hydrogen abstraction by five radicals (H, O( 3 P), OH, CH 3 , and HO 2 ) from methyl acetate (MA) is investigated theoretically in order to gain further understanding of certain aspects of the combustion chemistry of biodiesels, such as the effect of the ester moiety. Here, we employ ab initio quantum chemistry methods, coupled cluster singles and doubles with perturbative triples correction (CCSD(T)) and multireference averaged coupled pair functional theory (MRACPF2), to predict chemically accurate reaction energetics. Overall, MRACPF2 predicts slightly higher barrier heights than CCSD(T) for MA + H/CH 3 /O/OH, but slightly lower barrier heights for hydrogen abstraction by HO 2 . Based on the obtained reaction energies, we also report high-pressure-limit rate constants using transition state theory (TST) in conjunction with the separable-hindered-rotor approximation, the variable reaction coordinate TST, and the multi-structure all-structure approach. The fitted modified Arrhenius expressions are provided over a temperature range of 250 to 2000 K. The predictions are in good agreement with available experimental results. Abstractions from both of the methyl groups in MA are expected to contribute to consumption of the fuel as they exhibit similar rate coefficients. Moreover, the reactions involving the OH radical are predicted to have the highest rates among the five abstracting radicals, while those initiated by HO 2 are expected to be the lowest.

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

Modeling the Behavior of Complex Aqueous Electrolytes Using Machine Learning Interatomic Potentials: The Case of Sodium Sulfate

Understanding the structure and thermodynamics of solvated ions is essential for advancing applications in electrochemistry, water treatment, and energy storage. While ab initio molecular dynamics methods are highly accurate, they are limited by short accessible time and length scales whereas classical force fields struggle with accuracy. Herein, we explore the structure and thermodynamics of complex monovalent-divalent ion pairs using Na 2 SO 4 (aq) as a case study by applying a machine learning interatomic potential (MLIP) trained on density functional theory (DFT) data. Our MLIP-based approach reproduces key bulk properties such as density and radial distribution functions of water. We provide the hydration structure of the sodium and sulfate ions in the 0.1–2 M concentration range and the one-dimensional and two-dimensional potentials of mean force for the sodium–sulfate ion pairing at the low concentration limit (0.1 M), which are inaccessible to DFT. At low concentrations, the sulfate ion is strongly solvated, leading to the stabilization of solvent-separated ion pairs over contact ion pairs. Minimum energy pathway analysis revealed that coordinating two sodium ions with a sulfate ion is a multistep process whereby the sodium ions coordinate to the sulfate ion sequentially. Finally, we demonstrate that MLIPs allow the study of solvated ions beyond simple monovalent pairs with DFT-level accuracy in their low concentration limit (0.1 M) via statistically converged properties from ns-long simulations.

anions↗
DOE OSTI2015

Reactivity at the Lithium–Metal Anode Surface of Lithium–Sulfur Batteries

Due to their high energy density and reduced cost, lithium–sulfur batteries are promising alternatives for applications such as electrical vehicles. However, a number of technical challenges need to be overcome in order to make them feasible for commercial uses. These challenges arise from the battery highly interconnected chemistry, which besides the electrochemical reactions includes side reactions at both electrodes and migration of soluble polysulfide (PS) species produced at the cathode to the anode side. The presence of such PS species alters the already complex reactivity of the Li anode. In this paper, interfacial reactions occurring at the surface of Li metal anodes due to electrochemical instability of the electrolyte components and PS species are investigated with density functional theory and ab initio molecular dynamics methods. It is found that the bis(trifluoromethane)sulfonimide lithium salt reacts very fast when in contact with the Li surface, and anion decomposition precedes salt dissociation. The anion decomposition mechanisms are fully elucidated. Two of the typical solvents used in Li–S technology, 1,3-dioxolane and 1,2-dimethoxyethane, are found stable during the entire simulation length, in contrast with the case of ethylene carbonate that is rapidly decomposed by sequential 2- or 4-electron mechanisms. Finally, on the other hand, the fast reactivity of the soluble PS species alters the side reactions because the PS totally decomposes before any of the electrolyte components forming Li 2 S on the anode surface.

25 ENERGY STORAGE↗
DOE OSTI2015

Electronic excitation induced amorphization in titanate pyrochlores: an ab initio molecular dynamics study

In this study, the response of titanate pyrochlores (A 2 Ti 2 O 7 , A = Y, Gd and Sm) to electronic excitation is investigated utilizing an ab initio molecular dynamics method. All the titanate pyrochlores are found to undergo a crystalline-to-amorphous structural transition under a low concentration of electronic excitations. The transition temperature at which structural amorphization starts to occur depends on the concentration of electronic excitations. During the structural transition, O 2 -like molecules are formed, and this anion disorder further drives cation disorder that leads to an amorphous state. This study provides new insights into the mechanisms of amorphization in titanate pyrochlores under laser, electron and ion irradiations.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Multivalent optical cycling centers: towards control of polyatomics with multi-electron degrees of freedom

Optical control of polyatomic molecules promises new opportunities in precision metrology and fundamental chemistry, as well as quantum information and many-body science. Contemporary experimental and theoretical efforts have mostly focused on cycling photons via excitation of a single electron localized to an alkaline earth (group 2)-like metal center. In this paper, we consider pathways towards optical cycling in polyatomic molecules with multi-electron degrees of freedom, which arise from two or more cycling electrons localized to p-block post-transition metal and metalloid (group 13, 14, and 15) centers. Here, we characterize the electronic structure and rovibrational branching of several prototypical candidates using ab initio quantum chemical methods. Despite increased internal complexity and challenging design parameters, we find several molecules possessing quasi-closed photon cycling schemes with highly diagonal, visible and near-infrared transitions. Furthermore, we identify new heuristics for engineering optically controllable and laser-coolable polyatomic molecules with multi-electron cycling centers. Our results help elucidate the interplay between hybridization, repulsion, and ionicity in optically active species and provide new directions for using polyatomic molecules with complex electronic structure as a resource for quantum science and measurement.

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

Alkali metal mediated C–C bond coupling reaction

Metal catalyzed carbon-carbon (C–C) bond formation is one of the important reactions in pharmacy and in organic chemistry. In the present study, the electron and hole capture dynamics of a lithium-benzene sandwich complex, expressed by Li(Bz){sub 2}, have been investigated by means of direct ab-initio molecular dynamics method. Following the electron capture of Li(Bz){sub 2}, the structure of [Li(Bz){sub 2}]{sup −} was drastically changed: Bz–Bz parallel form was rapidly fluctuated as a function of time, and a new C–C single bond was formed in the C{sub 1}–C{sub 1}′ position of Bz–Bz interaction system. In the hole capture, the intermolecular vibration between Bz–Bz rings was only enhanced. The mechanism of C–C bond formation in the electron capture was discussed on the basis of theoretical results.

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

Melting of sodium under high pressure. An ab-initio study

We report ab-initio molecular dynamics simulations of dense liquid/solid sodium for a pressure range from 0 to 100 GPa. The simulations have been performed with the orbital free ab-initio molecular dynamics method which, by using the electron density as the basic variable, allows to perform simulations with large samples and for long runs. The calculated melting curve shows a maximum at a pressure ≈ 30 GPa and it is followed by a long, steep decrease. These features are in good agreement with the experimental data. For various pressures along the melting curve, we have calculated several liquid static properties (pair distribution functions, static structure factors and short-range order parameters) in order to analyze the structural effects of pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

On the calculation of complete dissociation curves of closed-shell pseudo-onedimensional systems via the complete active space method of increments

The method of increments (MoI) has been employed using the complete active space formalism in order to calculate the dissociation curve of beryllium ring-shaped clusters Be{sub n} of different sizes. Benchmarks obtained through different quantum chemical methods including the ab initio density matrix renormalization group were used to verify the validity of the MoI truncation which showed a reliable behavior for the whole dissociation curve. Moreover we investigated the size dependence of the correlation energy at different interatomic distances in order to extrapolate the values for the periodic chain and to discuss the transition from a metal-like to an insulator-like behavior of the wave function through quantum chemical considerations.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Ab-initio simulations of higher Miller index Si:SiO{sub 2} interfaces for fin field effect transistor and nanowire transistors

Models of three representative higher Miller index interfaces, Si(310):SiO{sub 2}, Si(410):SiO{sub 2}, and Si(331):SiO{sub 2}, have been built by an ab-initio molecular dynamics method. We show that each interface can be made as a fully bonded network without any defects and has a reasonable electronic structure for use in fin field effect transistors or gate-all-around nanowire devices. The differences in numbers of oxygen bridges are attributed to the intermediate sub-oxide components and the atomic step structure. The interface bonding schemes to passivate different densities of dangling bonds on different facets are also analyzed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Applications of the modified Hulthén-Kohn method for bound and scattering states

We adapt the Hulthén–Kohn method suggested by Efros [Phys. Rev. C 99, 034620 (2019)] for calculating various observables in the continuum and discrete spectrum using two-body interactions in single- and coupled channel systems. We explore the convergence of phase shifts and wave functions as well as the location of S-matrix poles which enables obtaining both resonance and bound state parameters. We find that employing a harmonic oscillator basis, together with an interaction smoothing scheme introduced by Gyarmati et al. [Nucl. Phys. A 326, 119 (1979)], and adopting approximate bound-state solutions for the short-range components of basis wave functions lead to good convergence even with restricted oscillator quanta accessible for modern no-core shell model codes. The adapted Efros method will facilitate ab initio many-body nuclear structure applications.

Nuclear reactions↗
DOE OSTI2020

Group-III quantum defects in diamond are stable spin-1 color centers

Color centers in diamond have emerged as leading solid-state “artificial atoms” for a range of quantum technologies, from quantum sensing to quantum networks. Concerted research activities are now underway to identify new color centers that combine stable spin and optical properties of the nitrogen vacancy (NV – ) with the spectral stability of the silicon vacancy (SiV – ) centers in diamond, with recent research identifying other group-IV color centers with superior properties. In this paper, we investigate a class of diamond quantum emitters from first principles, the group-III color centers, which we show to be thermodynamically stable in a spin-1, electric-field-insensitive structure. Further, from ab initio electronic structure methods, we characterize the product Jahn-Teller (pJT) effect present in the excited-state manifold of these group-III color centers, where we capture symmetry-breaking distortions associated with strong electron-phonon coupling. These predictions can guide experimental identification of group-III vacancy centers and their use in applications in quantum information science and technology.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Simulating contrast inversion in atomic force microscopy imaging with real-space pseudopotentials

Atomic force microscopy (AFM) measurements have reported contrast inversions for systems such as Cu 2 N and graphene that can hamper image interpretation and characterization. In this work, we apply a simulation method based on ab initio real-space pseudopotentials to gain an understanding of the tip-sample interactions that influence the inversion. We find that chemically reactive tips induce an attractive binding force that results in the contrast inversion. We find that the inversion is tip height dependent and not observed when using less reactive CO-functionalized tips.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Electronic structure and surface properties of MgB 2 (0001) upon oxygen adsorption [Electronic structure and surface properties of MgB2: oxygen adsorption on MgB2(0001)]

Here, we use density-functional theory to investigate the bulk and surface properties of MgB 2 . The unique bonding structure of MgB 2 is investigated by Bader's atoms-in-molecules, charge density difference, and occupancy projected band structure analyses. Oxygen adsorption on the charge-depleted surfaces of MgB 2 is studied by a surface potential energy mapping method, reporting a complete map including low-symmetry binding sites. The B-terminated MgB 2 (0001) demonstrates reconstruction of the graphenelike B layer, and the reconstructed geometry exposes a threefold site of the subsurface Mg, making it accessible from the surface. Detailed reconstruction mechanisms are studied by simulated annealing method based on ab initio molecular dynamics and nudged elastic band calculations. The surface clustering of B atoms significantly modifies the B 2p states to occupy low energy valence states. The present paper emphasizes that a thorough understanding of the surface phase may explain an apparent inconsistency in the experimental surface characterization of MgB 2 . Furthermore, these results suggest that the surface passivation can be an important technical challenge when it comes to development of a superconducting device using MgB 2 .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗