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Implementation of analytical excited state gradients for open-shell systems in time-dependent density functional theory plus tight binding (TDDFT+TB) method
Excited state potential energy surfaces are essential for understanding molecular behavior in excited states, including electronic transitions, reaction dynamics, and other photochemical and photophysical properties. When excited state minima or reaction dynamics are of interest, the gradients of the total excited state energy are required. Time-dependent density functional theory (TDDFT) is a widely used method, as it provides reasonably accurate excited state energies compared to experiment; however, its computational cost scales unfavorably with molecule size. To address this limitation, methods that maintain TDDFT accuracy while reducing the computational cost are needed. The time-dependent density functional theory plus tight binding (TDDFT+TB) method offers comparable accuracy to TDDFT with significantly improved efficiency. Previously, Havenridge et al. implemented TDDFT+TB analytical excited state gradients for closed-shell molecules. This was achieved using a Lagrangian-based approach by taking analytical derivatives of the excitation energies with respect to nuclear coordinates. In this study, we extend this implementation to handle open-shell molecular systems. Our unrestricted TDDFT+TB analytical gradient implementation follows the framework of the unrestricted TDDFT analytical gradient code in the Amsterdam Density Functional engine within the Amsterdam Modeling Suite. Building on the existing closed-shell implementation, we focus on key modifications to the coupling matrix and the incorporation of spin indices in the gradient expressions required for open-shell systems. We validate the method through calculations on various molecular systems, comparing gradient accuracy, emission energies, and computational cost to demonstrate that it delivers reliable results for open-shell molecular excited states with improved efficiency compared to the standard TDDFT method.
Hamiltonian formulation of field–transmission line coupling and its classical–quantum correspondence
A Hamiltonian formulation of electromagnetic field coupling to transmission-line systems is presented, providing a unified framework that connects classical field–conductor interaction models with their quantum mechanical description. Starting from the distributed circuit representation, the electromagnetic coupling is incorporated through canonical variables, allowing the total system to be expressed in terms of a well-defined Hamiltonian. The resulting equations of motion reproduce the established classical field-to-line coupling relations in the appropriate limit, while simultaneously enabling a consistent quantization of the transmission-line dynamics. This approach clarifies the physical meaning of the coupling terms, establishes the correspondence between classical field excitation and quantum operators, and provides access to quantum features, such as vacuum fluctuations and single-mode excitations. The formulation does not rely on phenomenological assumptions, making it applicable to a wide range of transmission-line configurations and frequency regimes. These results offer a physically transparent bridge between classical electromagnetic theory and quantum-enabled transmission-line systems, with potential relevance to quantum circuits, waveguide-based devices, and electromagnetic interaction modeling at the quantum–classical interface.
Random phase approximation-based local natural orbital coupled cluster theory
Practical applications of fragment embedding and closely related local correlation methods depend critically on a judicious choice of low-level theory to define the local embedding subspace and to account for long-range electrostatic and correlation effects outside the embedding region. Second-order Møller–Plesset perturbation theory (MP2) is by far the most widely used correlated low-level theory; however, its applicability becomes questionable for systems in which MP2 is known to fail, either quantitatively or qualitatively. In this work, we present the random phase approximation (RPA) and the closely related second-order screened exchange (SOSEX) as promising alternatives to MP2 within the local natural orbital-based coupled-cluster (LNO-CC) framework. Through benchmark calculations on noncovalent molecular complexes and crystals, reaction barrier heights, and bulk metals, we demonstrate that RPA- and SOSEX-based LNO-CC at the LNO-coupled-cluster singles doubles triples [CCSD(T)] level closely match the performance of the corresponding MP2-based approach, while at the LNO-CCSD level they yield significantly faster convergence toward the canonical CCSD limit. The improvement is particularly pronounced for metallic systems as the thermodynamic limit is approached. These results highlight the critical role of the low-level theory in fragment embedding and local correlation methods and identify RPA as a compelling alternative to the commonly used MP2.
Computing the thermal transport properties of three-dimensional discontinuous systems with interfacial thermal resistance
This study proposes a practical method based on the step-balanced random walk for evaluating thermal transport properties in complex three-dimensional discontinuous systems incorporating interfacial thermal resistance (ITR). The random walk preserves detailed balance by using the ratio of thermal effusivities—representing the material’s ability to exchange energy—across interfaces as the penetration probability. ITR is introduced by assigning each interface a thermal effusivity bI smaller than that (b) of the constituent phases. Applications to two representative models—a two-phase slab structure and a spherical filler dispersion—demonstrate that the proposed approach predicts physically consistent dependencies of thermal conductivity on interfacial density and filler size. Furthermore, the ITR per unit area, RI, can be expressed through a simple relation involving b, bI, and the base step length Δr0: RI = const. × Δr0 (b − bI)/bI.
Modeling the flow-driven disassembly and extensional rheology of end-linking telechelic polymers
End-linking telechelic polymers can self-assemble into supramolecular chains that act as high-molecular-weight viscosity modifiers in linear response, but disassemble rather than degrade in nonlinear flows. We use coarse-grained molecular dynamics to simulate the nonequilibrium dynamics of end-linking linear telechelic polymers with bivalent associations during nonlinear elongational flows. Steady state flow curves reveal a near-perfect rate-thinning of the steady state extensional viscosity ηE∼ε̇−1, which coincides with a plateau in the steady-state extensional stress σE. This stress plateau coincides with an unexpected decrease in average chain extension with increasing strain rate, which arises from enhanced stretch fluctuations driven by flow-induced bond dissociation and reformation. Flow drives the disassembly of large supramolecular chains, but the distribution of supramolecular weights remains exponentially distributed. The average molecular weight of supramolecular chains decreases with increasing rate with ⟨Mc⟩∼ε̇−1/2, resulting in diffusive scaling of the extensional viscosity, with the average supramolecule size ηE∼⟨Mc⟩2.
Bidirectional micropump driven by oscillating water droplet-shaped microbubbles via acoustic actuation
The driving and distribution of fluid is important for building integrated and functional microfluidic systems. This study presents a bidirectional micropump based on acoustically oscillating water droplet-shaped microbubbles. The micropump exploits the geometric asymmetry of the microbubbles along the pumping direction to generate asymmetric acoustic microstreaming actuated with acoustic waves, enabling directional fluid transport. Bidirectional pumping is achieved by adjusting the frequency of actuated acoustic waves, which switches the direction of the acoustic microstreaming. The prototypes were fabricated, and experimental tests were conducted. Experimental results show that water droplet-shaped microbubbles generate out-of-plane microstreaming near the base ends when the frequency of the actuated acoustic waves is between 11.85 and 13.11 kHz. As a result of that, the net flow from the base end to the tip of the water droplet-shaped microbubbles is formed by the superposition of the array of acoustic microstreaming. The micropump achieved its peak forward flow of 2658 nl/min at an acoustic frequency of 12.32 kHz and 60 Vpp. A reverse out-of-plane microstreaming is generated at the middle of the microbubbles when the frequency of acoustic waves is within the range of 18.22–19.35 kHz. A reverse flow from the tip to the base end is generated by the superposition of acoustic microstreaming produced by the microbubble array. A peak reverse flow of 1230 nl/min was achieved at 18.55 kHz and 140 Vpp. This study proposes a flexible bidirectional micropump, offering a solution for the dynamic regulation of flow direction in microfluidic systems.
Concurrently coupling particle and continuum simulations to study block copolymer membrane fabrication
We present a concurrent multiscale simulation framework for membrane fabrication from a block copolymer solution via self-assembly and nonsolvent-induced phase separation, combining simulations of a soft, coarse-grained particle model with the continuum Uneyama–Doi model. The computationally intensive particle model provides a molecularly resolved description of micro- and macrophase separation, including thermal fluctuations, while the computationally efficient continuum model captures the process-driven self-assembly of nonequilibrium membrane morphologies on large length and time scales. Central to the approach is a machine learning-guided adaptive coupling strategy, implemented through a coordinator library, which predicts the evolving spatiotemporal subdomain where high-fidelity particle simulations are required and dynamically allocates computational resources accordingly. The two complementary models are concurrently coupled through a consistent exchange of the solvents’ fluxes, enabling the treatment of spatially inhomogeneous, multicomponent systems with diffusive transport, micro- and macrophase separation, and vitrification. This adaptive strategy enables predictive simulations of membrane formation on experimentally and technologically relevant length and time scales, reaching micrometers and minutes. As an application, we examine the influence of polymer concentration in the initial casting solution. The framework is general and extensible, providing a computational tool for investigating nonequilibrium structure formation in complex multicomponent soft-matter systems.
Surface-potential-based metal-oxide-semiconductor field-effect transistor compact model with localized states down to 4.2 K toward cryogenic applications
This paper presents a new surface-potential-based cryo-MOSFET compact model that incorporates multiple carrier sources and remains valid across a wide temperature (T) range from 300 K down to 4.2 K. By explicitly accounting for doping concentration, incomplete ionization, and localized states, a unified analytical surface potential expression is derived via Schroder series. The proposed model captures the T-dependent variation of charge transport under cryogenic conditions and introduces the concept of an effective temperature (Teff), achieving excellent agreement with experimental data. In addition, key cryo-T device behaviors, including drain-induced barrier lowering, subthreshold-slope (SS) saturation, time-dependent reliability (i.e., bias-temperature-instability), and high electric-field (E) induced kink, are accurately reproduced. Furthermore, 6T-SRAM cells with different W/L configurations are analyzed and simulated at 300, 77, and 4.2 K. The results show significant improvements in both the read and write static noise margins at 4.2 K, demonstrating the potential applicability of this compact model for basic cryo-circuit design, particularly in applications where localized-state-induced non-idealities are prominent, such as in peripheral circuitry for quantum computing systems.
Radial gausslets
Gausslets are one of the few examples of basis sets for electronic structure that allow for two-index/diagonal electron–electron interaction terms. A weakness of gausslets is that, because of their 1D origin, they have been tied to Cartesian coordinates. Here, we generalize the gausslet construction for the radial coordinate in three dimensions for atomic basis sets. These radial gausslets make a very compact radial basis with a relatively modest number of functions, with diagonal interaction terms. We illustrate the accuracy of this construction with Hartree–Fock and exact diagonalization on atomic systems.
Direct measurement of reflection loss and electromagnetic parameter retrieval in high-loss materials
Electromagnetic interference shielding (EMIS) and microwave absorption (MA) are two ways to tackle the deteriorating electromagnetic pollution. Generally, they present contradictory electromagnetic behavior, and it is challenging to simultaneously achieve both EMIS and MA properties within a single material. Thus, highly absorbed EMIS (HA-EMIS) material is a compromising solution. However, the electromagnetic parameter calculation method of MA is unsuitable for HA-EMI materials, severely hindering the unification of EMIS and MA. To address this issue, we propose both a new theoretical method for extracting the electromagnetic parameters of non-magnetic high-loss materials and a direct measurement technique for MA property. The validity of the proposed approach is verified through both simulations and experiments. These results indicate that high EMIS materials with excellent MA property are expected to be developed in the near future under the guidance of our work, facilitating the integration of these two research areas.
In search of density functional approximations for accurate evaluation of NMR shielding constants for main-group nuclei: Analytic derivatives, benchmarking, and insights from density-corrected density functional theory
Predicting nuclear magnetic resonance (NMR) chemical shielding constants (CSCs) remains a great challenge. Being second-order response properties, NMR CSCs impose stringent demands on the accuracy and approximations of theoretical methods, as they are highly sensitive to subtle changes in the electronic structure. In this work, we develop and implement analytic derivatives for gauge-including atomic orbital shielding tensors within a density-corrected density functional theory (DC-DFT) Lagrangian/Z-vector framework. We benchmark the results against the comprehensive NS372 set of main-group CSCs and analyze the method errors in view of density-driven and functional-driven components within the DC-DFT framework, thereby rationalizing when and why substituting the electron density improves NMR shielding predictions. We find that the commonly employed HF-density strategy yields only modest and highly nuclear-dependent improvements in CSC accuracy. In contrast, combining MN15-L electron density with the SCAN0 energy functional can lead to a substantial error cancellation, reducing the overall mean absolute deviation to 5.02 ppm, approaching the accuracy of state-of-the-art double-hybrid functionals while avoiding the computational cost of perturbative correlation treatments. Our results establish DC-DFT as a practical and interpretable strategy for enhancing NMR shielding predictions with lower-rung functionals and offer clear guidance for selecting beneficial density/energy functional pairings for magnetic response properties.
Conduction mechanisms in ferroelectric wurtzite Al1− <i>x</i> Hf <i>x</i> N heterovalent alloys
Characteristic electrical transport analysis of textured ferroelectric wurtzite Al1−xHfxN (x = 0.00, 0.07, 0.14, 0.28, 0.42) thin films was performed using temperature-dependent leakage current measurements on Mo/Al1−xHfxN/Mo/SiC capacitors. A unified analysis couples (i) slope-magnitude comparisons against the theoretical forms for conduction mechanisms of Poole–Frenkel, Schottky, and fixed-range hopping, (ii) extractions of effective energy barriers via Arrhenius fits with variable applied E-field when applicable, and (iii) low-field ohmic fits. Low-field response is ohmic for all x, while compositions with increasing x show an increase in the E-field value at which ohmic-to-non-ohmic onset occurs. The unified analysis shows a trend of high-field electrical transport shifting from interface limited (Schottky) to bulk limited (Poole–Frenkel) conduction with increasing x. A resulting conduction mechanism map illustrates this gradual composition-driven change from interface to bulk limited. This is consistent with HfAl acting as deep trap states, though mechanistic details require additional investigation.
The role of charge distribution in the modeling of polyatomic ions: The nitrate anion case
Nitrate salts are widely used in agriculture, industrial chemistry, and food preservation. Using computer simulations based on the intermolecular parameters of the extended Madrid-2019 force field [V. M. Trejos et al., J. Chem. Phys. 159(22), 224501 (2023)], we examine the effect of intramolecular partial charge distributions of the nitrate anion on the temperature of maximum density (TMD) of aqueous NaNO3, KNO3, and NH4NO3 solutions. A specific internal charge distribution improves agreement with experimental TMD data relative to the original force field and enables predictions for systems lacking experimental measurements. Densities are largely insensitive to the charge distribution, while transport properties (diffusion coefficients and viscosities) show only minor variations. In contrast, structural properties are sensitive to the choice of the charge distribution. The original force field provides better agreement with neutron diffraction structural data than the model yielding the most accurate TMD. An intermediate charge distribution is therefore selected as a compromise, providing a balanced description of all properties. Empirical force fields cannot reproduce all properties simultaneously; however, an appropriate parameter selection, including the intramolecular charge distribution as an additional degree of freedom, yields a consistent description of thermodynamic, transport, and structural properties.
Gate–drain leakage enhanced by drain-induced dielectric barrier lowering in gate-all-around field-effect transistors
Gate-All-Around Field-Effect Transistors (GAAFETs), now entering high-volume production as successors to fin field-effect transistor technology, are enabling continued scaling and enhanced performance in advanced semiconductor nodes. However, the drain current in GAAFETs strongly deviates from the thermionic dependence at negative gate voltages, exhibiting the existence of leakage that is additionally enhanced at high applied drain biases. Understanding the origin of this leakage is essential for determining the scaling limits of GAAFETs and for guiding device and material optimizations aimed at suppressing the off-state current. Additionally, recent experimental measurements have revealed the increased influence of radiation-induced defects in the negative gate-voltage regime, with their impact remaining largely negligible for positive gate voltages. Through predictive first-principles simulations, we demonstrate that the observed leakage current at negative gate voltages originates from gate-to-drain tunneling, which is significantly enhanced by drain-induced dielectric barrier lowering between the gate and drain.
Unraveling the dual nature of covalent and electrostatic binding in americyl capture by a phenanthroline-based covalent organic framework (DAPhen)
The separation of minor actinides, especially americium, from lanthanides in spent nuclear fuel remains a critical challenge in nuclear waste management, primarily due to their nearly identical chemical behavior in the trivalent state. To address this, we target the linear dioxo configuration of pentavalent americium (AmO2+), which offers distinct steric and electronic features compared to spherical trivalent lanthanides. This work investigates Am(V) adsorption using a phenanthroline-based covalent organic framework (DAPhen-COF), where the pre-organized N,O-donor environment from the phenanthroline-amidine motif is designed for strong actinide coordination. Multiscale computations show that DAPhen-COF forms a highly stable complex with AmO2+. Density of states analysis reveals strong orbital hybridization between Am-5f and ligand N/O-2p states, underscoring substantial covalent interaction. Topological analysis of electron density confirms the existence of bonds with pronounced covalent character within highly polarized coordination environments, while electrostatic potential analysis verifies complementary electrostatic contributions. Energy decomposition analysis further quantifies the binding as a cooperative interplay between orbital and electrostatic forces. Quantitative adsorption energy calculations further corroborate these findings, revealing that DAPhen-COF exhibits a strong affinity for Am(V) (−151.9 kcal/mol) and clear selectivity over Eu(III), with the most stable configuration arising from cooperative actinide-actinide interactions within the confined COF interlayer space. This study not only sheds light on the unique coordination chemistry of pentavalent americium but also provides a robust theoretical foundation for designing ligand architectures capable of distinguishing actinides from lanthanides based on oxidation-state-specific motifs.
Negative-bias temperature instability in AlSiO/p-type GaN metal-oxide-semiconductor field-effect transistors with ultrathin AlN interfacial layer
A technique for inserting a thin crystalline AlN interfacial layer (AlN-IL) at the oxide/p-type GaN interface is investigated to enhance the channel mobility in GaN metal-oxide-semiconductor field-effect transistors (MOSFETs). In this study, the negative-bias temperature instability (NBTI) for AlSiO/p-type GaN MOSFETs with and without an AlN-IL is discussed. Capacitance–voltage analyses revealed a high density of interface states at energy levels of 1 and 0.8 eV above the valence-band maximum for GaN in the MOSFETs with and without an AlN-IL, respectively. In the MOSFET without an AlN-IL, the threshold voltage (Vth) rapidly shifted to negative and subsequently remained unchanged, even when a stress voltage was applied. In addition, the shift was reduced at high temperatures, likely because of thermal emission of holes from the interface states. By contrast, the Vth shift for the MOSFET with an AlN-IL gradually increased with increasing applied bias stress time. When the AlN-IL thickness was reduced to 0.8 nm, the Vth shift decreased compared with that for the MOSFET without an AlN-IL. However, the negative Vth shift was enhanced and proportional to the logarithm of the stress time at a temperature of 100 °C, indicating a tunneling process. The increase in the Vth shift at 100 °C is likely caused by hole tunneling to the band discontinuity at AlSiO/AlN through the polarization-field-induced triangular potential in the AlN-IL. Thus, engineering of band alignment is a key for NBTI control in an AlSiO/AlN/p-type GaN MOSFET with high channel mobility.
Non-exponential relaxation without dynamic heterogeneity in van der Waals liquids above the melting point
We investigate the origin of relaxation stretching in van der Waals liquids above the melting point by means of depolarized dynamic light scattering. To this end, we study optically anisotropic probe molecules both in the bulk and when diluted in an optically isotropic solvent. Strikingly, the relaxation shape of the probe molecules in dilution is indistinguishable from that of the pure liquid composed of the probe molecules. By contrast, when explicit dynamic heterogeneity is introduced through a distribution of probe molecule sizes, the relaxation shape becomes sensitive to the solvent concentration. These findings indicate that dynamic heterogeneity has a negligible influence on the diverse relaxation shapes observed above the melting point, which seem to arise purely from intrinsic properties of the molecules such as anisotropic rotation and internal degrees of freedom. The observed behavior differs greatly from what has been reported close to the glass transition temperature, where both heterogeneous dynamics and a generic spectral shape of structural relaxation indicate a subordinate role of intrinsic molecular properties.
Temperature diagnosis of underwater microsecond high-current pulsed discharge plasma
Underwater pulsed arc discharge plasma serves as the “tool” for generating shock waves and high-energy species. To deeply study the characteristics of plasma channels, this paper conducted temperature diagnosis of underwater microsecond high-current pulsed discharge. A spectral measurement system calibration method considering the water absorption effect was proposed. Time-resolved emission spectra of the plasma channel were obtained, and the spectral characteristics during the arc discharge phase were analyzed. A diagnostic approach based on the combined analysis of spectral-line profiles and intensities was developed to determine the excitation temperature and blackbody radiation temperature. The time-dependent evolution of the blackbody radiation temperature and the excitation temperature of the plasma was characterized. The results indicate that, for kA-level pulsed high-current underwater arc plasmas, the temperature ranges of the order of 1–2 × 104 K.
Synchrotron x-ray diffraction study of liquid and glassy toluene
We measured the X-ray diffraction data of liquid toluene at several temperatures between 300 and 180 K and those of glasses prepared by liquid quenching and vapor deposition (VD) at 3 K. The VD glasses annealed at 90 and 112 K were also measured at 3 K. Throughout the measurements, we used a custom-made cryostat for VD experiments and a high-energy X-ray diffractometer at BL04B2, SPring-8. The structure factor S(Q) of each state was successfully fitted using potential fitting with molecular dynamics simulation and reverse Monte Carlo methods. Interestingly, in both the liquid and glassy states, the most preferred intermolecular structure is not a parallel one, which is the most stable dimer in vacuo, but a T-shaped one in which two hydrogen atoms of a benzene ring point toward the benzene ring of a neighboring molecule. The abundance of the T-shaped and side-by-side structures increases with decreasing temperature and annealing of the VD glass. As one possibility, we propose that the T-shaped structure can constitute the cooperatively rearranging region conceived in the Adam–Gibbs theory.
Atomistic insights into carbon cluster decomposition in amorphous SiOC under O-, N-, and NO-containing conditions: A molecular dynamics study
Carbon-related defects formed during thermal oxidation degrade SiC MOS device performance and reliability. NO annealing mitigates these defects, but the separate roles of oxygen and nitrogen in carbon-cluster decomposition and carbon removal at the SiC/SiO2 interface remain unclear. In this work, we perform melt-quench molecular dynamics simulations of amorphous SiOC derived from a Si30O60C12 reference model, adding up to 12 O or N atoms or 6 NO pairs, to track C—C network fragmentation and gas-like CO/CO2 unit formation. Carbon clusters decreased under all added conditions, but redistribution was species-dependent. O addition increased the fraction of carbon atoms in the gas category, i.e., CO/CO2-like units detached from the Si-containing network, to about 40%, while the solid phase retained a well-connected C—C network. In contrast, N addition caused pronounced solid-phase fragmentation of the C—C bond network, with only about 10% of carbon classified into this gas category. Under the N-added condition, among solid-phase carbon atoms, about 55% had no C—C bonds and about 40% were twofold coordinated. NO addition showed an intermediate gas-category carbon fraction of about 30%, whereas fragmented states with nitrogen-bonded low-coordination carbon remained common. The C—N distance distribution showed a short-bond component at 1.2–1.3 Å, suggesting multiple-bond character. This strong C—N bonding is associated with the stabilization of low-coordination carbon and the reduction of C—C bonding in the solid phase. Overall, these findings suggest that nitrogen and oxygen play complementary roles under NO annealing, with nitrogen fragmenting solid-phase carbon and oxygen promoting the formation of gas-like CO/CO2 precursor units relevant to carbon removal at the SiC/SiO2 interface.