Previous Calculations Research Articles

Self-Organization of Square Nitrogen Islands on Copper (100) Surfaces: Island Pair Distributions Based on an Ab-Initio DFT and Mesoscopic Interaction Model.

Earlier scanning tunneling microscopy (STM) studies have shown that nitrogen forms square c(2×2) islands on Cu(100) surfaces, the assembly of which depends on coverage. Recent calculations have revealed that adatom-adatom interactions are compatible with the formation of square c(2×2) islands. The pair distribution of these islands is a topic of the current investigation. In addition, the domain structure of islands found recently in STM experiments is discussed and explained using two types of island interactions. The potential used for the interaction between N square islands on copper is a combination of previous ab initio calculations and an updated mesoscopic interaction scheme. The 2D-BGY integral equation is extended to handle binary objects like two different types of islands. It is solved for up to a quarter island coverage using a numerical recursion algorithm. Coverage beyond half is accessible with island vacancy symmetry. The calculation predicts the formation of ordered island assemblies with increasing coverage. These results reasonably explain the previous experimental observations. The handling of binary objects also seems to be applicable to assemblies of two adatom types as found in catalysis. The assumptions and limitations of the model are discussed and open questions are formulated.

Hydrogen Atom Abstraction Reaction from Silane with Hydrogen and Methyl Radicals: Rate Constants and Kinetic Isotopic Effects.

The thermal rate constants and H/D kinetic isotope effects (KIEs) of the reactions SiH4 + H SiH3 + H2 and SiH4 + CH3 SiH3 + CH4 were calculated with the variational transition state theory including multidimensional tunneling corrections (VTST-MT). The ωB97X-D and CCSD(T) methods were employed to compute the barrier height and reaction energy. The ωB97X-D/aug-cc-pVTZ methodology was used to build reaction paths, and the CCSD(T)/CBSQ-5 approach was used to improve the energetics of the stationary points in the calculations of CVT/μOMT thermal rate constants. For the SiH4 + H pathway, the CVT/μOMT rate constants and H/D KIEs are in excellent agreement with previous experimental and theoretical calculations; otherwise, for the SiH4 + CH3 pathway, which has few experimental and theoretical data available, it has provided, for the first time, rate constants and kinetic isotope effects using variational transition state theory with multidimensional tunneling corrections.

Growth and magnetic properties of epitaxial thin films of the [formula omitted]-MAX phase (Mn[formula omitted]Sc[formula omitted])[formula omitted]GaC

i-MAX phases are quaternary variants of the nanolaminated MAX phases, with additional in-plane ordering of the M atoms. The combination of in-plane and out-of-plane ordering potentially gives rise to complex magnetic behaviour. The i-MAX phase (Mn2/3Sc1/3)2GaC has been synthesized in epitaxial thin film form on three different substrates, SiC-4H(001), MgO(111) and Al2O3(0001), by magnetron sputtering using elemental targets. Structural characterization by x-ray scattering and scanning transmission electron microscopy confirms the phase on all three substrates, although the highest crystal quality is obtained on SiC-4H(001). High-resolution images reveal the distinctive i-MAX structure, which is orthorhombic of space group Cmcm. Magnetic characterization reveals that the ground state is most likely antiferromagnetic. This confirms previous theoretical calculations which predicted an antiferromagnetic ground state and establishes the (Mn2/3Sc1/3)2GaC i-MAX phase as a potential candidate for antiferromagnetic spintronic applications.

X-Ray Spectra of Black Hole X-Ray Binaries with Returning Radiation

In the disk–corona model, the X-ray spectrum of a stellar-mass black hole in an X-ray binary is characterized by three components: a thermal component from a thin and cold accretion disk, a Comptonized component from a hot corona, and a reflection component produced by illumination of the cold disk by the hot corona. In this paper, we assume a lamppost corona, and we improve previous calculations of the X-ray spectrum of black hole X-ray binaries. The reflection spectrum is produced by the direct radiation from the corona as well as by the returning radiation of the thermal and reflection components and is calculated considering the actual spectrum illuminating the disk. If we turn the corona off, the reflection spectrum is completely generated by the returning radiation of the thermal component, as it may happen for some sources in soft spectral states. After choosing the radial density profile of the accretion disk, the ionization parameter is calculated self-consistently at any radial coordinate of the disk from the illuminating X-ray flux and the local electron density. We show the predictions of our model in different regimes, and we discuss its current limitations as well as the next steps to improve it.

Theoretical analysis and predictions for the two-neutrino double electron capture of 124Xe

Abstract We provide a complete theoretical description of the two-neutrino electron capture in 124Xe, improving both the nuclear and the atomic structure calculations. We improve the general formalism through the use of the Taylor expansion method, leading to higher-order terms in the decay rate of the process. The nuclear part is treated with pn-QRPA and interacting shell model (ISM) methods. The nuclear matrix elements (NMEs) are calculated with the pn-QRPA method with isospin restoration by fixing the input parameters so that the experimental decay rate is reproduced, resulting in values significantly lower than in previous calculations. The validity of the pn-QRPA NMEs is tested by showing their values to be comparable with the ones for double-beta decay with the emission of two antineutrinos of 128,130Te, which have similar pairing features. Within the ISM, we reproduce the total experimental half-life within a factor of two and predict the capture fraction to the KK channel of about 74%. We also predict the capture fractions to other decay channels and show that for the cumulative decay to the KL1–KO1 channels, a capture fraction of about 24% could be observed experimentally. On the atomic side, calculations are improved by accounting for the Pauli blocking of the decay of innermost nucleon states and by considering all s-wave electrons available for capture, expanding beyond the K and L1 orbitals considered in previous studies. We also provide improved atomic relaxation energies of the final atomic states of 124Te, which may be used as input for background modeling in liquid Xenon experiments.

Ka-Band Rotational Spectroscopy of Succinimide and N-Chlorosuccinimide.

Succinimide and its derivatives are cyclic five-membered rings that appear in a variety of natural products and are widely used in organic synthesis. From a structural standpoint, succinimide contains an NH group in the ring which interacts with two adjacent carbonyl groups, pushing the ring structure toward planarity at the expense of increasing ring strain and eclipsing interactions among the out-of-plane hydrogen atoms in the two CH2 groups. Previous quantum chemical calculations at different levels of theory have predicted both a nonplanar C2 structure and a planar C2v structure, the latter of which is the most consistent with gas-phase electron diffraction measurements. Here, we report the pure rotational spectra of succinimide and N-chlorosuccinimide in the 26.5-40.0 GHz range using chirped-pulse Fourier transform microwave spectroscopy, supported by coupled cluster and density functional theory quantum chemical calculations. The spectra were fit to Watson's A-reduced effective Hamiltonian, including both 35Cl and 37Cl isotopologues of N-chlorosuccinimide as well as the N and Cl quadrupole hyperfine interactions. On the basis of the agreement with quantum chemical calculations and the measured inertial defects, we find that the rotational spectra are consistent with a planar ring structure, with a maximum out-of-plane angle of ≤5°.

Understanding the Mid-Infrared Spectra of Protic Ionic Liquids by Density Functional Theory.

Protic ionic liquids (PILs) are promising candidates as electrolytes for proton exchange polymer membrane fuel cells. In order to optimize their properties, a detailed understanding of the molecular interactions within the bulk and at the electrode-electrolyte interface is needed, which can be obtained by infrared spectra. A prerequisite for extracting information on the molecular structure and inter- or intramolecular interactions from an experimental spectrum is a reasonable interpretation of the observed spectral features. Here, we employed density functional theory to understand the vibration modes of PILs composed of ammonium cations and different counteranions. Different from the previous calculation methods performed on small cluster model systems consisting of isolated species, a periodically repeated system of four ion pairs was used in order to approximate the bulk liquid environment. The computed frequencies and IR intensity match well with the corresponding experimental spectra, allowing for its proper interpretation, especially the characteristic features of the interionic interaction. The presented approach enables accurate computation of a variety of ionic liquid systems in a highly efficient way.

Simple Recurrent Networks are Interactive.

There is disagreement among cognitive scientists as to whether a key computational framework - the Simple Recurrent Network (SRN; Elman, Machine Learning, 7(2), 195-225, 1991; Elman, Cognitive Science, 14(2), 179-211, 1990) - is a feedforward system. SRNs have been essential tools in advancing theories of learning, development, and processing in cognitive science for more than three decades. If SRNs were feedforward systems, there would be pervasive theoretical implications: Anything an SRN can do would therefore be explainable without interaction (feedback). However, despite claims that SRNs (and by extension recurrent neural networks more generally) are feedforward (Norris, 1993), this is not the case. Feedforward networks by definition are acyclic graphs - they contain no loops. SRNs contain loops - from hidden units back to hidden units with a time delay - and are therefore cyclic graphs. As we demonstrate, they are interactive in the sense normally implied for networks with feedback connections between layers: In an SRN, bottom-up inputs are inextricably mixed with previous model-internal computations. Inputs are transmitted to hidden units by multiplying them by input-to-hidden weights. However, hidden units simultaneously receive their own previous activations as input via hidden-to-hidden connections with a one-step time delay (typically via context units). These are added to the input-to-hidden values, and the sums are transformed by an activation function. Thus, bottom-up inputs are mixed with the products of potentially many preceding transformations of inputs and model-internal states. We discuss theoretical implications through a key example from psycholinguistics where the status of SRNs as feedforward or interactive has crucial ramifications.

Pion photoproduction of nucleon excited states with Hamiltonian effective field theory

We refine our previous calculation of multipole amplitude E0+ for pion photoproduction process, γN→πN. The treatment of final-state interactions is based upon an earlier analysis of pion-nucleon scattering within Hamiltonian effective field theory, supplemented by incorporating contributions from the N*(1650) and the KΛ coupled channel. The contribution from the bare state corresponding to the N*(1650) significantly enhances our results. Additionally, we also compute the multipole amplitude M1−, which is of direct relevance to the Roper resonance. The results are comparable with other dynamical coupled-channel models, even though the contribution from the bare state (interpreted as a 2s excitation) in this channel is small because of its large mass. Published by the American Physical Society 2024

Detection of a Planar Tetracoordinate Hydrogen within the Indium Framework by Quantum Dynamics Theory.

In this work, we consider the question how to detect the planar tetracoordination hydrogen geometry which was recently proposed by electronic structure calculations on the In4H+ system (Angew. Chem. Int. Ed. 2024, 63, e202317312; e202400927; e202403214). Keeping the C4v symmetry, a two-dimensional model of In4H+ is designed to build the nonadiabatic Hamiltonian operator with the lowest-lying singlet and triplet states coupled with spin-orbit coupling. The electronic energies in fitting the potential energy matrix are computed at either the MRCI or CCSD (T) level. Having constructed Hamiltonian, the multiconfigurational time-dependent Hartree product method predicts vibrational eigenstates for spectrum and recrossing probability of the proton. These quantum dynamics calculations predict a period of ∼60 fs for the proton recrossing the In4 moiety and further indicate that experimental observation of the D4h geometry for the triplet state is highly probable at room temperature even though the present MRCI and previous CASPT2 calculations (Angew. Chem. Int. Ed. 2024, 63, e202400927) both predict the C4v symmetry. To measure the C4v geometry, a low temperature of ≲30 K could be adopted. Despite the low temperature, the experiment might still miss the C4v geometry due to the nonzero recrossing probability of H at the low kinetic energy region. Further, a new type of hydrogen bonding in the D4h geometry is proposed to explain the interaction between the C4v geometries.

Surface structure of Sn doped β-Ga2O3(010) p(1×1) studied by quantitative low energy electron diffraction

We have studied the surface structure of a single crystal β-Ga2O3(010) using quantitative Low Energy Electron Diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The XPS measurements show spectra typical of stoichiometric Ga2O3 with a clean surface. LEED consistently shows a p(1×1) pattern, free of surface reconstruction. Quantitative LEED I(V) curves are acquired for 41 distinct diffraction spots. The experimental I(V) curves are compared to simulations over the first five layers. The best fits to the experimental LEED I(V) curves acquired at all diffraction spots are then used to calculate the interplanar relaxation and atomic rumpling. Significant atomic rumpling and interplanar relaxation are found over the first 5 atomic layers. As a result of rumpling a polarization of ∼ 2 µC/cm2 develops in the topmost surface layer. The structural results are in good agreement with previous density functional theory calculations and experimental X-ray photoelectron diffraction.

When LAMOST meets Gaia DR3. Exploring the metallicity of open clusters

Open clusters (OCs) are excellent probes as their age and abundance can be tightly constrained, allowing us to explore the distribution of metallicity and composition across the disk of the Milky Way. By conducting a comprehensive analysis of the metallicity of OCs, we can obtain valuable information about the history of their chemical enrichment. Moreover, by observing stars in different regions of the Milky Way, we can identify significant spatial structures in their chemical composition and abundance. This enables us to understand stellar birth radii through chemical tagging. Nevertheless, it remains challenging to infer the original positions of OCs using current data alone. The aim of this study is to investigate the distribution of metallicity in the solar neighborhood using a large dataset from Gaia DR3 combined with LAMOST spectra. With accurate ages and metallicity measurements, we can determine birth radii for the stars and attempt to understand their migration pattern. We chose a total of 1131 OCs within 3 Kpc of the Sun from the Gaia DR3 and LAMOST DR8 low-resolution spectral database (R=1800). We used an artificial neural network to correct the LAMOST data by incorporating high-resolution spectral data from GALAH DR3 (R=28000). The average metallicity of the OCs was determined based on the reliable Fe/H values for their members. We then examined the distribution of metallicity across different regions within the Galaxy and inferred birth radii of the OCs from their age and metallicity. The correction method presented here can partially eliminate the systematic offset for LAMOST data. We discuss the metallicity trend as a function of Galactocentric distance and the guiding radii. We also compare these observational results with those from chemo-dynamic simulations. Values derived from observational metallicity data are slightly lower than predicted values when the uncertainties are not considered. However, the metallicity gradients are consistent with previous calculations. Finally, we investigated the birthplace of OCs and find hints that the majority of OCs near the Sun have migrated from the outer Galactic disk.

Analysis of the Principle for Gravitational Wave Detection Based on Pulsar Timing Arrays

As a matter of fact, the hunt for gravitational waves have been under the spotlight since LIGO researchers took home the Nobel Prize. In reality, different from the laser interferometers used by LIGO, pulsar timing arrays are also feasible ways to detect gravitational waves. With this in mind, this article will mainly focus on how pulsar timing arrays could detect gravitational waves as well as its achievements so far. To be specific, pulsar timing arrays consisted of rotating pulsars in space, and their regular burst of radio waves serve as stopwatches. When gravitational waves pass by, the frequency at which human beings’ receiving radio waves will change at the same time. Pulsar timing array projects around the world have been busy recording pulsars up to now, but more discoveries will come out soon. They also utilized their data to verify previous calculations such as the Helling-Downs curve. According to the analysis, the current limitations and prospects are demonstrated.

Modified plastic stress distribution method incorporating size effect for strength prediction of CFST members

Large-size concrete-filled steel tubular (CFST) members are widely applied in long-span and heavily-loaded structures due to increasing engineering demands. However, due to the size effect, applying the existing design methods to large-size CFST members may overestimate the load capacity, leading to unsafe designs. The plastic stress distribution (PSD) method is widely used as the primary method in typical design codes for calculating the strength of CFST members that are not susceptible to local buckling, while the PSD method was mainly used previously to evaluate the strength of CFST members with relatively small size. In this work, the experimental database of short CFST members with compact sections was complied, and the applicability of the PSD method was evaluated. The results showed that the test-to-calculated ratios decreased with increasing size of CFST members, and unsafe designs emerged for large-size CFST members. Based on the observations, the mechanics-based modified plastic stress distribution (MPSD) method was proposed, which not only considered the size effect at the material level similar to the way treated in most of the previous calculation methods, but also incorporated the size effect of the interaction action between the steel tube and concrete at the member level. Finally, the MPSD method was evaluated against the experimental database, and good agreement was achieved. These results provide strong justification for changes to the PSD method in design standards.

A Reversible Perspective on Petri Nets and Event Structures

Event structures have emerged as a foundational model for concurrent computation, explaining computational processes by outlining the events and the relationships that dictate their execution. They play a pivotal role in the study of key aspects of concurrent computation models, such as causality and independence, and have found applications across a broad range of languages and models, spanning realms like persistence, probabilities, and quantum computing. Recently, event structures have been extended to address reversibility, where computational processes can undo previous computations. In this context, reversible event structures provide abstract representations of processes capable of both forward and backward steps in a computation. Since their introduction, event structures have played a crucial role in bridging operational models, traditionally exemplified by Petri nets and process calculi, with denotational ones, i.e., algebraic domains. In this context, we revisit the standard connection between Petri nets and event structures under the lenses of reversibility. Specifically, we introduce a subset of contextual Petri nets, dubbed reversible causal nets , that precisely correspond to reversible prime event structures. The distinctive feature of reversible causal nets lies in deriving causality from inhibitor arcs, departing from the conventional dependence on the overlap between the postset and preset of transitions. In this way, we are able to operationally explain the full model of reversible prime event structures.

Electrostatic energy of solid binary ionic mixtures.

We study the electrostatic energy of binary ionic mixtures (BIMs) in the form of Coulomb crystals with the main focus on ordered crystals. We consider 15 different binary bcc-like lattices, accurately calculate their electrostatic energies, and approximate them by a unified equation. These results extend those available in the literature. A detailed comparison with selected previous results is made, particularly, using previous calculations in the linear mixing rule approximation. The case of disordered BIMs is also outlined. The results are expected to be useful for exploring multicomponent Coulomb systems in compact stars, laboratory plasmas, and technological applications.

Ferric iron stabilization at deep magma ocean conditions.

Fe2O3 produced in a deep magma ocean in equilibrium with core-destined alloy sets the early redox budget and atmospheric composition of terrestrial planets. Previous experiments (≤28 gigapascals) and first-principles calculations indicate that a deep terrestrial magma ocean produces appreciable Fe3+ but predict Fe3+/ΣFe ratios that conflict by an order of magnitude. We present Fe3+/ΣFe of glasses quenched from melts equilibrated with Fe alloy at 38 to 71 gigapascals, 3600 to 4400 kelvin, analyzed by synchrotron Mössbauer spectroscopy. These indicate Fe3+/ΣFe of 0.056 to 0.112 in a terrestrial magma ocean with mean alloy-silicate equilibration pressures of 28 to 53 gigapascals, producing sufficient Fe2O3 to account for the modern bulk silicate Earth redox budget and surficial conditions near or more oxidizing than the iron-wüstite buffer, which would stabilize a primitive CO- and H2O-rich atmosphere.

Dynamical friction in dark matter spikes: Corrections to Chandrasekhar’s formula

We consider the intermediate mass-ratio inspiral of a stellar-mass compact object with an intermediate-mass black hole that is surrounded by a dark matter density spike. The interaction of the inspiraling black hole with the dark matter particles in the spike leads to dynamical friction. This can alter the dynamics of the black hole binary, leaving an imprint on the gravitational wave signal. Previous calculations did not include in the evaluation of the dynamical friction coefficient the contribution from particles that move faster than the inspiraling black hole. This term is neglected in the standard Chandrasekhar treatment where only slower moving particles contribute to the decelerating drag. Here, we demonstrate that dynamical friction produced by the fast moving particles can have a significant effect on the evolution of a massive binary within a dark matter spike. For a density profile ρ∝r−γ with γ≲1, the dephasing of the gravitational waveform can be several orders of magnitude larger than estimated using the standard treatment. As γ approaches 0.5 the error becomes arbitrarily large. Finally, we show that dynamical friction tends to make the orbit more eccentric for any γ<1.8. However, energy loss by gravitational wave radiation is expected to dominate the inspiral, leading to orbital circularization in most cases. Published by the American Physical Society 2024

Determination of the rigidity of the geomagnetic cutoff and simulation of the motion of particles in the Earth’s magnetosphere

A method for determination of the geomagnetic cutoff rigidity is presented. The method is based on the tracing of charged particles in the Earth’s magnetic field using Buneman-Boris’ particle-in-cell method. The results of the verification of the method are presented: in particular, a comparison with theoretical calculations in an ideal dipolar field and with previous calculations made under the real field condition. The developed method has shown a high reliability proven by the replication of the known effects. In the dipolar approximation, it has shown high accuracy in comparison with the theoretical calculations. Typical pattern of geomagnetic cutoff penumbra is also reproduced.

First-Principles Linear Combination of Atomic Orbitals Calculations of K2SiF6 Crystal: Structural, Electronic, Elastic, Vibrational and Dielectric Properties.

The results of first-principles calculations of the structural, electronic, elastic, vibrational, dielectric and optical properties, as well as the Raman and infrared (IR) spectra, of potassium hexafluorosilicate (K2SiF6; KSF) crystal are discussed. KSF doped with manganese atoms (KSF:Mn4+) is known for its ability to function as a phosphor in white LED applications due to the efficient red emission from Mn⁴⁺ activator ions. The simulations were performed using the CRYSTAL23 computer code within the linear combination of atomic orbitals (LCAO) approximation of the density functional theory (DFT). For the study of KSF, we have applied and compared several DFT functionals (with emphasis on hybrid functionals) in combination with Gaussian-type basis sets. In order to determine the optimal combination for computation, two types of basis sets and four different functionals (three advanced hybrid-B3LYP, B1WC, and PBE0-and one LDA functional) were used, and the obtained results were compared with available experimental data. For the selected basis set and functional, the above-mentioned properties of KSF were calculated. In particular, the B1WC functional provides us with a band gap of 9.73 eV. The dependencies of structural, electronic and elastic parameters, as well as the Debye temperature, on external pressure (0-20 GPa) were also evaluated and compared with previous calculations. A comprehensive analysis of vibrational properties was performed for the first time, and the influence of isotopic substitution on the vibrational frequencies was analyzed. IR and Raman spectra were simulated, and the calculated Raman spectrum is in excellent agreement with the experimental one.