Coupling Calculations Research Articles

Nuclear Magnetic Resonance Gas-Phase Studies of Spin-Spin Couplings in Molecules

This paper overviews gas phase experiments with respect to one fundamental part of nuclear magnetic resonance (NMR) spectra. Indirect spin-spin coupling is an important parameter of NMR spectra and is observed as the splitting of spectral signals. A molecule containing two different magnetic nuclei (e.g., hydrogen HD, HT, or DT) exhibits this interaction in an external magnetic field measured as the spin-spin coupling parameter, nJ(NN′). Modern quantum chemical methods allow the precise calculation of spin-spin coupling, but it is never easy because nJ(NN′) is modified by temperature and intermolecular interactions. Accurate calculations can be performed only for small isolated molecules. NMR spectroscopy can deliver measurements of spin-spin couplings for isolated molecules if nJ(NN′) parameters are observed in the gas phase as a function of density. The extrapolation of such measurements to the zero-density limit permits nJ0(NN′) determination free from intermolecular interactions. The latter technique can also be applied to liquid vapors in molecules like acetonitrile or water. Spin-spin couplings across one chemical bond (1J0(NN′)) are the largest and most important for theoretical modeling. The present review reports numerous 1J0(NN′) parameters recently measured by multinuclear NMR spectra of gaseous samples.

The validity of physical optics for microwave analysis: accuracy through simplicity

Abstract Traditionally, the optics and microwave communities have been separated not only by the THz band but also by differences in technical knowledge, analytical and computational approaches, design processes, and measurement methodologies. In this contribution, we aim to bridge this divide by leveraging a computational asymptotic technique from optics, known as physical optics, which holds significant potential for the microwave/antenna community. This technique allows for precise microwave calculations for radiation patterns, reflection coefficients, material losses, and mutual coupling between antennas.

Exploring superconductivity in dynamically stable carbon-boron clathrates trapping molecular hydrogen

Abstract The recent discovery of type-VII boron–carbon clathrates with calculated superconducting transition temperatures approaching ~100 K has sparked interest in exploring new conventional superconductors that may be stabilized at ambient pressure. The electronic structure of the clathrate is highly tunable based on the ability to substitute different metal atoms within the cages, which may also be large enough to host small molecules. Here we introduce molecular hydrogen (H2) within the clathrate cages and investigate its impact on electron–phonon coupling interactions and the superconducting transition temperature (Tc). Our approach involves combining molecular hydrogen with the new diamond-like covalent framework, resulting in a hydrogen-encapsulated clathrate, (H2)B3C3. A notable characteristic of (H2)B3C3 is the dynamic behavior of the H2 molecules, which exhibit nearly free rotations within the B–C cages, resulting in a dynamic structure that remains cubic on average. The static structure of (H2)B3C3 (a snapshot in its dynamic trajectory) is calculated to be dynamically stable at ambient and low pressures. Topological analysis of the electron density reveals weak van der Waals interactions between molecular hydrogen and the B–C cages, marginally influencing the electronic structure of the material. The electron count and electronic structure calculations indicate that (H2)B3C3 is a hole conductor, in which H2 molecules donate a portion of their valence electron density to the metallic cage framework. Electron–phonon coupling calculation using the Migdal–Eliashberg theory predicts that (H2)B3C3 possesses a Tc of 46 K under ambient pressure. These results indicate potential for additional light-element substitutions within the type-VII clathrate framework and suggest the possibility of molecular hydrogen as a new approach to optimizing the electronic structures of this new class of superconducting materials.

Structural Optimization and Performance Evaluation of Liquid Cooled Super Fast Charging Cable Based on Multi-physics Coupling Calculation

Structural Optimization and Performance Evaluation of Liquid Cooled Super Fast Charging Cable Based on Multi-physics Coupling Calculation

Study on finite element method coupled with genetic algorithm in optimization for temperature uniformity of hot plate welding

Aiming to improve the temperature uniformity used for hot plate welding, two optimal design methods by coupling finite element method (FEM) with genetic algorithm (GA) are put forward and implemented into FEM software through the secondary development. Taking the positions of the heating holes and the heat fluxes applying to the heating holes as the optimization variables, the minimum temperature difference on the faying surface as the optimization objective, taking advantage of the numerical calculation of steady-state heat transfer, the parametric modeling of hot plate is carried out in accordance with the developed FEM-GA coupling optimization framework. The first optimal design method adopts a two-stage sequent optimization that the optimal positions of heating holes and the heat fluxes applying to the heating holes are sequentially obtained by using the two-step FEM-GA coupling calculations. The second optimal design method optimizes these variables simultaneously. The simultaneous optimal design method is further utilized to optimize the hot plate model associated with the orthogonal heating holes. In the case of the experience-based initialization of the optimization parameters, the two-stage sequent optimal design method is capable to achieve the better temperature uniformity on the faying surfaces with the lower computation costs, as compared with the results of the simultaneous optimal design method. The FEM-GA coupling optimal design method for improving the temperature uniformity used for hot plate welding would be of great significance for the industrial application of hot plate welding.

Design of High‐Tc Quaternary Hydrides Under Moderate Pressures Through Substitutional Doping

AbstractIn recent years, there has been considerable interest in exploring high‐temperature superconductors in hydrides at low‐pressure conditions. A series of quaternary XAZ2H16 compounds (X, A = La, Ca, Sr, Ba, Ac, Ce, Th, Na or K; Z = Be, B, Si or P) are constructed based on ternary Fm‐3m LaB/BeH8‐type structure. Eight hydrides are found to be thermodynamically stable, with LaThBe2H16 and AcBaSi2H16 exhibiting lower pressures of 92 and 81 GPa, respectively. All investigated hydrides display dynamical stability at pressures below one million atmospheres, with LaThBe2H16 remaining stable even as low as 8 GPa. Moreover, LaSrB2H16, AcBaSi2H16, AcSrSi2H16, and Ba2SiPH16 exhibit clear covalent bonding characteristics between Z and H atoms, while Be atoms tend to form ionic bonds with H atoms. Electron‐phonon coupling calculations indicate that all of these quaternary hydrides have potential as superconductors. LaThBe2H16, AcBaSi2H16, AcSrSi2H16 and YCeBe2H16 exhibit superconducting transition temperatures (Tcs) of 126, 137, 123, and 104 K at 10, 30, 30, and 45 GPa, respectively, while LaYBe2H16 exhibits the highest Tc of 221 K at 75 GPa. These findings provide valuable insights for the search for high‐Tc superconductors under lower pressure conditions and encourage further exploration of polynary superconducting hydrides.

Ground States for Metals from Converged Coupled Cluster Calculations.

Many-electron correlation methods offer a systematic approach to predicting material properties with high precision. However, practically attaining accurate ground-state properties for bulk metals presents significant challenges. In this work, we propose a novel scheme to reach the thermodynamic limit of the total ground-state energy of metals using coupled cluster theory. We demonstrate that the coupling between long-range and short-range contributions to the correlation energy is sufficiently weak, enabling us to restrict long-range contributions to low-energy excitations in a controllable way. Leveraging this insight, we calculated the surface energy of aluminum and platinum (111), providing numerical evidence that coupled cluster theory is well-suited for modeling metallic materials, particularly in surface science. Notably, our results exhibit convergence with respect to finite-size effects, basis-set size, and coupled cluster expansion, yielding excellent agreement with experimental data. This paves the way for more efficient coupled cluster calculations for large systems and a broader utilization of theory in realistic metallic models of materials.

Numerical study of specific C-shaped heat exchanger using porous medium and coarse mesh coupling method

Porous medium method is an effective simplified approach for the heat exchanger simulation when the numerous tubes and complex structures bring unacceptable computational resources to full-scale simulation. However, for the specific C-shaped heat exchanger, the tube side resistance and flow distribution characteristics that significantly affects the heat transfer and natural circulation performance cannot be obtained by the traditional porous medium method coupled with the one-dimensional node model for tube side. It is urgent to develop a new coupling method that can not only simplify modeling and calculation, but also obtain tube side high-resolution flow heat transfer characteristics. This study presents a novel calculation method which couples the porous medium mesh and the tube side coarse mesh. The coupling method was then applied to a typical C-shaped passive residual heat removal heat exchanger (PRHR HX), which is significant to the stable operation and the mitigation of accident conditions in the nuclear power plant. The coupling calculation method was validated against the full-scaled simulation and data from scaled-down experiment. The average relative error of heat transfer power is 2.1 % under steady state conditions. Under transient conditions, the average absolute error of the fluid temperature and wall temperature are less than 6 K. Then the 3D thermal-hydraulic characteristics of PRHR HX was studied. Owing to a greater gravitational pressure head, the mass flow rates in the extended tubes surpass those in the shorter tubes by a relative deviation of 27.4 %. The upper horizontal tube's heat exchange capacity constitutes approximately 70 % of the total power, leading to enhanced natural circulation. The outlet temperatures from various tubes tend to equalize. The boiling area gradually extended down along the heat transfer tubes. Our work would present an innovative calculation method to the numerical study and design optimization of the C-shaped heat exchanger.

Study on the dynamic characteristics of the revolver cannon under high-speed impact

Abstract Aiming at the problems of discontinuous mechanism action, excessive energy consumption and poor matching with the chainless feeding system under the harsh working conditions of high-speed strong impact, a certain revolver cannon has the problems of discontinuous mechanism action, excessive energy consumption and poor matching with the chainless feeding system. In order to further study the dynamic characteristics of the gas-operated revolver cannon under high-speed impact load, solve the matching problem between the revolver cannon and the chainless feeding system, and improve the firing rate of the revolver cannon, based on the principle of the revolver cannon, the multi-body dynamics and gas dynamics theory are used to couple the thermomechanical coupling calculation model of the revolver cannon gas chamber and the transmission efficiency and impact numerical calculation method between the key parts into the ADAMS dynamic simulation model, and the dynamic analysis and calculation of the whole cannon of the revolver cannon are carried out. The motion characteristic curves of the key parts of the revolver cannon are obtained by simulation, the motion matching between the key parts is analyzed, and the simulation results are verified by experiments. The results show that the error of the motion characteristic parameters such as the active slide displacement and the gun box recoil displacement obtained by simulation and experiment is very small, which verifies the correctness of the calculation model. The simulation calculation method can provide technical support for the matching research between the revolver cannon and the chainless feeding system and the design and structural optimization of the revolver cannon.

ANSYS Modelling of Piezoelectric Smart Beam using Ritz Vector Method and Sub-structuring Analysis

Abstract Finite element analysis (FEA) is an important tool for smart structure modelling, particularly in the context of piezoelectric beams. A key challenge in such modelling is the derivation of electro-elastic coupling, which explains how these structures respond mechanically to electrical loading. Traditional methods rely on intricate shape functions that encompass both electrical and mechanical degrees of freedom, rendering the process complex. In this study, an innovative approach—the so-called Ritz vector method— which facilitates modelling of the system based on loading effect is implemented to simplify electro-elastic coupling calculations. The method previously applied in MSC Nastran with a thermal analogy system and validated experimentally is now implemented in the widely accessible ANSYS software. By harnessing ANSYS’ robust piezoelectric capabilities and sub-structuring analysis for matrix extraction, the essential piezoelectric coupling and dielectric matrices are derived for piezoelectric stack and patch actuator driven cantilevered beams. Subsequently, state space models of these smart structures are constructed using MATLAB and results are compared with ANSYS FEA static and dynamic response. Active Disturbance Rejection Control (ADRC) was implemented on the modelled beams and results show significant suppression of free vibration due to impulse excitation of the tip. This work portrays the potential of the Ritz vector method, a powerful yet underexplored tool that simplifies electro-elastic coupling calculations for piezoelectric smart structures.

Prediction of transient thermal stress distribution in SOFC based on coupled computational fluid dynamics and thermodynamics modeling

Long-term stability and safety are two key factors for the application of solid oxide fuel cell (SOFC) technology, with thermodynamics playing a central role in influencing this stability. Thus, understanding the thermodynamic behavior within SOFC and how it will depend on the stack structure are very important, especially at the start and stop stages. In this study, a 3D calculated fluid dynamics model and thermomechanical coupling model based on the actual component structures are established. We place emphasis on understanding how these factors evolve during dynamic phases, shedding light on the transient thermal stress behavior of the SOFC. The influence of different interconnect structures on the thermal stress distribution is studied. The coupling calculation results show that the first principal stress of the electrolyte is significantly affected by the specific interconnect structure. Adopting cylindrical ribs instead of rectangular ribs, the thermal stresses of the SOFC components can be reduced by 13–25 %, respectively.

Dynamic Response Assessment of Floating Offshore Wind Turbine Mooring Systems with Different In-Line Tensioner Configurations Based on Fully Coupled Load Calculations

In-line tensioning technology has significantly reduced the cost barriers that previously hindered the expansion of the floating offshore wind industry. However, assessing the impact of in-line tensioners on the dynamic response of floating offshore wind turbines (FOWTs) lacks effectiveness, and the relevant mooring configuration specifications are not complete. Thus, a fully coupled calculation method is introduced in this paper to solve the relevant issues in mooring systems with in-line tensioners using a classic spar platform model. Three distinct design scenarios were selected to study the variation in mooring configurations of in-line tensioners along different mooring lines and at varied positions within each line. The potential occurrence of reverse tension phenomena was deliberated and assessed. We identified the varying tension patterns at the fairlead and in-line tensioner locations in mooring systems with in-line tensioners, and the influence of such variations on platform dynamics. The findings also demonstrate that the appropriate configuration of in-line tensioners should be selected to avoid the risk of reverse tension. This research has potential to contribute to the security and economy of the deployment of this emerging in-line mooring method.

Crystal Structure Design and Synthesis of Noncentrosymmetric Superconductors in Intercalation-Induced Transition Metal Dichalcogenides.

In this paper, we have performed a crystal structure screening and properties prediction framework within the noncentrosymmetric AMX2 system, which arises from the intercalation of elements in transition metal dichalcogenides. After rigorous evaluations of thermodynamic and dynamic stability, we have refined our initial structure pool of 504 crystals to a focused set of 48 promising candidates. Analysis of their electronic properties has revealed that 23 of these crystals exhibit semiconducting behavior. The results on electron-phonon coupling and band calculations indicate that most of the remaining 25 crystals are superconducting topological nodal line or Weyl semimetals. Additionally, we find that β-InNbSe2 in the AMX2 family can serve as an excellent candidate to explore topological nodal lines and Lifshitz transitions. Furthermore, we also extended the thermodynamic stability analysis to higher temperatures. Ultimately, we have successfully synthesized β-InNbSe2 with a stoichiometric ratio of 1:1:2 experimentally and characterized its superconducting properties. This study represents a systematic foray into the identification of new superconducting and topological materials within the noncentrosymmetric AMX2 family. This screening framework can be extended to other quasi two-dimensional systems.

Theoretical Study on Singlet Fission Dynamics and Triplet Migration Process in Symmetric Heterotrimer Models.

Singlet fission (SF) is a photophysical process where one singlet exciton splits into two triplet excitons. To construct design guidelines for engineering directional triplet exciton migration, we investigated the SF dynamics in symmetric linear heterotrimer systems consisting of different unsubstituted or 6,13-disubstituted pentacene derivatives denoted as X/Y (X, Y: terminal and center monomer species). Time-dependent density functional theory (TDDFT) calculations clarified that the induction effects of the substituents, represented as Hammett's para-substitution coefficients σp, correlated with both the excitation energies of S1 and T1 states, in addition to the energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). Electronic coupling calculations and quantum dynamics simulations revealed that the selectivity of spatially separated TT states for heterotrimers increased over 70%, superior to that in the homotrimer: an optimal region of the difference in σp between the substituents of X and Y for the increase in SF rate was found. The origin of the rise in SF rate is explained by considering the quantum interference effect: reduction in structural symmetry opens new interaction paths, allowing the S1-TT mixing, which contributes to accelerating the hetero-fission between the terminal and center molecules.

Exciton Enhanced Nonlinear Optical Responses in Monolayer h-BN and MoS2: Insight from First-Principles Exciton-State Coupling Formalism and Calculations.

Excitons are vital in the photophysics of materials, especially in low-dimensional systems. The conceptual and quantitative understanding of excitonic effects in nonlinear optical (NLO) processes is more challenging compared to linear ones. Here, we present an ab initio approach to second-order NLO responses, incorporating excitonic effects, that employs an exciton-state coupling formalism and allows for a detailed analysis of the role of individual excitonic states. Taking monolayer h-BN and MoS2 as two prototype 2D materials, we calculate their second harmonic generation (SHG) susceptibility and shift current conductivity tensor. We find strong excitonic enhancement in the NLO responses requires that the resonant excitons are not only optically bright themselves but also able to couple strongly to other bright excitons. Our results explain the occurrence of two strong peaks in the SHG of monolayer h-BN and why the A and B excitons of MoS2 unexpectedly exhibit minimal excitonic enhancement in both SHG and shift current generation.

Quantum rotational dynamics of linear C5 at low interstellar temperatures for H2 collision.

The quantum dynamics of carbon chains through H2 and He collisions in the interstellar medium (ISM) is an important step toward accurate modeling of their abundance in non-local thermodynamic equilibrium conditions. The C5(Σg+1) molecule is the longest pure carbon chain detected in the ISM to date. While He collisions are computationally easy to perform, the collision with much more abundant H2 is both complicated and computationally demanding. Using templates for approximating p-H2 collisional rates, such as scaling He rates and using a reduced 4D → 2D potential energy surface (PES), has limited applicability. On the other hand, any such approximation does not exist for o-H2. Therefore, a full rotational dynamics of C5 with both p- and o-H2 is performed considering both molecules as rigid-rotors. The PES is calculated using CCSD(T)-F12a/AVTZ, and a neural network fitting model has been carefully chosen to strictly obey spectroscopic accuracy and augment the PES. The augmented PES is then expanded into radial terms using the bispherical harmonics function, and close coupling calculations have been done to get the cross sections and, subsequently, rate coefficients for various rotational transitions of C5.

High-throughput structural screening and property study of noncentrosymmetric superconductors in Th-based ZrNiAl-like compounds

Noncentrosymmetric superconductors (NCSs) are emerging as potential materials for the investigation of nontraditional phenomena, including both the presence of a spin-singlet pairing configuration and the occurrence of a spin-triplet pairing configuration, as well as the manifestation of topological superconductivity. In this work, we constructed a high-throughput theoretical design and performed a computational screening of Th-based NCSs with ZrNiAl-like structures based on the first principles method. Through systematic stability evaluations, we expanded the range of Th-based ZrNiAl-like compounds with superior stability to 45 materials. In this group of substances, we have pinpointed 17 prospective entities exhibiting notable band separation in the vicinity of the Fermi level. Electron–phonon coupling calculations indicated that 14 of these compounds are superconducting. By performing an analysis of electronic properties, we proposed that ThAlRu and ThAlOs are two promising NCSs in this family. This work may aid in the exploration of the design and synthesis of Th-based ZrNiAl-like materials, leading to the discovery of more NCSs with unique superconducting properties.

Preliminary Implementation of High-Resolution Multi-Scale coupling calculations for the entire pressure vessel based on OpenFOAM

Significant coupling effects exist among system components in nuclear pressure vessel. Due to the complex geometric structures, the nuclear industry primarily relies on system codes or sub-channel methods for core safety analysis. However, these methods suffer from low model accuracy and insufficient coupling capabilities. Additionally, the differences in model scales impede direct coupling analysis with the CFD calculations of the plenum system. To address these issues, this paper proposes a multi −scale coupling calculation method for the entire pressure vessel: For the plenum system, detailed CFD modeling is employed, while the core calculations are conducted using CorTAF, a high-resolution core Multiphysics-coupling analysis method developed by our team. A cross-resolution coupling model is utilized to integrate the two, achieving cross-resolution coupling simulations for the entire pressure vessel, encompassing both the plenum and core system. The above method was applied to the coupling calculations of a typical pressurized water reactor’s lower plenum and core, revealing detailed thermal–hydraulic phenomena under precise core flow inlet distribution conditions. The lateral flow at the core inlet exceeds 1 m/s, with the maximum and minimum fluid velocities in the subchannels deviating by up to 70 % from the average velocity of 2.42 m/s. The flow distribution only begins to stabilize after a height of 1.2 m in the core. The paper also includes inlet asymmetric flow reduction calculations. Overall, the method enables multi-scale and multi-physics coupling calculations, which provide significant reference value for improving the accuracy of current core safety analyses.

Reduced Scaling Correlated Natural Transition Orbitals for Multilevel Coupled Cluster Calculations.

Multilevel coupled cluster theory offers reduced scaling computation of intensive properties in systems that are too large for standard coupled cluster calculations. A significant benefit of the multilevel coupled cluster framework is the possibility of calculating intensive properties that are not tightly localized if an appropriate set of active orbitals is used. Correlated natural transition orbitals (CNTOs) are tailored to describe excitation processes. For multilevel coupled cluster singles and doubles (MLCCSD) and singles and perturbative doubles (MLCC2) calculations, the construction of CNTOs generally becomes the computational bottleneck. Here, we demonstrate how CNTOs can be obtained with operations, eliminating the -scaling steps involved in the original approach. This reduction in scaling moves the bottleneck of MLCC2 and MLCCSD calculations from the active orbital space preparation to the MLCC2 and MLCCSD equations with -scaling.

Radiative transfer of Lyman-α photons at cosmic dawn with realistic gas physics

Abstract Lyman-α photons enable the cosmic dawn 21-cm signal through a process called the Wouthuysen-Field effect. An accurate model of the signal in this epoch hinges on the accuracy of the computation of the Lyα coupling, which requires one to calculate the specific intensity of Lyα photons emitted from the first stars. Most traditional calculations of the Lyα coupling assume a delta-function scattering cross-section, as the resonant nature of the Lyα scattering makes an accurate radiative transfer (RT) solution computationally expensive. Attempts to improve upon this traditional approach using numerical RT have recently emerged. However, some of these treatments suffer from assumptions such as a uniform gas distribution, coherent scattering in the gas frame and isotropic scattering. While others which do not account for these only do so through certain schemes along with core-skipping algorithms. We present results from a self-consistent Monte Carlo RT simulations devoid of any of the assumptions in the previous work for the first time. We find that gas bulk motion is the most important effect to account for in RT resulting in an RMS difference of 37% in the 21-cm signal and anisotropic scattering being the least important effect contributing to less than 3% RMS difference in 21-cm signal. We also evaluate the 21-cm power spectrum and compare that with the traditional results at cosmic dawn. This work points the way towards higher-accuracy models to enable better inferences from future measurements.