Browse Articles

Spin crossover in metal–organic frameworks: A crystal embedded multi-reference study

Spin crossover (SCO) in transition metal (TM)-containing solid state materials remains a challenge for the electronic structure calculations as some of the electronic states may have a significant multi-reference character. The periodic effective Hamiltonian of crystal field (pEHCF) method accurately describes strong correlations in TM-containing crystalline systems. In this work, pEHCF has been applied to study the electronic structure of the high spin and low spin states in the Fe(pyridine)2Ni(CN)4 metal–organic framework (MOF). The relative energy of the spin states involved in SCO has been calculated, and the degeneracy line exhibiting a strong dependence on the distance between an Fe ion and the CN groups has been identified. The degeneracy line also displays a step-like dependence on the position of the pyridine ligands in the narrow interval of 2.08–2.10 Å, while outside this interval, the dependence is weak. Low-temperature paramagnetism of the Fe(pyridine)2Ni(CN)4 SCO-MOF has been explained by the triplet ground state of Ni in the square-planar coordination with the CN groups. The electronic structure of a recently synthesized Fe2(H0.67bdt)3 SCO-MOF has been also investigated. This MOF contains two types of Fe ions and exhibits unusual spin crossover behavior above room temperature. Our calculations confirm that in the temperature range of 300–423 K, Fe2 ions undergo a spin transition from quintet (S = 2) to singlet (S = 0), while Fe1 ions exist in the low-spin configuration in both initial (300 K) and final structures (423 K).

Publisher Correction: The impact of pre-existing aortic stenosis and mitral regurgitation on patients with acute myocardial infarction

Self-assembly and non-equilibrium phase coexistence in a binary granular mixture

We report the experimental observation of a square crystalline phase in a vibrated binary mixture of spherical grains. This structure spontaneously forms from a disordered state, consistently with predictions obtained in an equilibrium system with similar geometrical properties under conservative dynamics. By varying the area fraction, we also observe stable coexistence between a granular fluid and an isolated square crystal. Using realistic simulations based on the discrete element method and an idealized collisional model integrated via event-driven molecular dynamics, we not only reproduce experimental results but also help to gain further insights into the non-equilibrium phase coexistence. Through the direct phase coexistence method, we demonstrate that the system shows behavior highly similar to an equilibrium first-order phase transition. However, the crystal remains at a higher granular temperature than the fluid, which is a striking non-equilibrium effect. Through qualitative arguments and supported by kinetic theory, we elucidate the role of the coupling between local structure and energy transfer mechanisms in sustaining kinetic temperature gradients across the fluid–solid interface.

Publisher Correction: Characterization of aroma profiles and microbial communities of cigar tobacco leaves from different varieties and origins and their correlations analysis

Molecular dynamics of ice-active solutions at ice–water interfaces

Small molecules that interact strongly with water were the subject of this molecular dynamics (MD) study. These solutes include a cryoprotectant (DMSO), a polyalcohol [CH2(OH)2], carboxylic acid conjugates (HCOOH and HCOONa), an ammonium salt (NH4Cl), and two alkyl halide salts (NaCl and NaF). MD simulations were carried out for bulk supercooled liquids and solutions in contact with ice. Solute and water hydrogen bonding, orientational and translational order, and hydrogen bond jump dynamics were compared in bulk and as a function of distance from the solute molecules. Reverse non-equilibrium molecular dynamics simulations were used to determine interfacial widths, friction coefficients (κ) with ice, and solution phase viscosities (η). Ionic solutes were found to reduce orientational and translational ordering near the ice interfaces. However, in bulk liquids, we find a correlation between orientational ordering and the statistics of water hydrogen bonds—a donor–acceptor imbalance in water has the greatest impact on ordering in the bulk liquids. Although ionic solutions exhibited similar effects on the water structure, the effect on dynamics depends most directly on donor–acceptor imbalance. Solutes that are hydrogen bond acceptors were found to slow hydrogen bond lifetimes relative to hydrogen bond donors. We also observed a direct correlation between the liquid phase hydrogen-bond jump times and shear viscosity. Finally, of all the solutes studied, only DMSO and sodium formate exhibited increased friction at the ice–water interface.

Frequency-multiplexed tunable logic device based on terahertz graphene-integrated metamaterial composed of two circular ring resonator array

An instantaneous voice-synthesis neuroprosthesis

Thermochemical properties of anhydrous crystals and molten alkali metal halide salts from molecular simulations of phase-transferable polarizable force fields

Alkali halides find application not only under standard thermodynamic conditions but also at elevated temperatures, for example, in molten salt reactors or heat transfer and storage in solar applications. This study presents the temperature dependence of the thermochemical properties of their salts at normal pressure and temperatures ranging from 298.15 K up to the boiling points of the salts. The values were obtained using molecular simulations with polarizable DLM/2022-BK3 force fields. In most cases, our results show excellent agreement with experiments and often similar or better predictive capability compared to the most accurate polarizable models available in the literature developed for simulations of anhydrous alkali halides. Relatively worse predictions are observed for salts in which very small anions strongly polarize large cations. Our results for density, energy, chemical potential, and heat capacity of melts and crystals, and also melting temperatures, confirm the excellent phase transferability of the force fields used. They also serve as a substitute for missing experimental data for rubidium and cesium halides and highlight inaccuracies in some experimental data for the densities of LiBr, NaF, and NaBr crystals found in the literature.

Comparison of combined femoral nail and plate fixation versus dual plate fixation in the treatment of AO/OTA 33C distal femoral fractures

Neuromorphic heat transport effects in a molecular junction

Understanding energy transport at the nanoscale is an open and fundamental challenge in the molecular sciences with direct implications for the design of new electronics, computing devices, and materials. While nanoscale energy transport under steady-state conditions has been studied extensively, there is much less known about energy transport under time-dependent driving forces, particularly in the far-from-equilibrium regime. In this work, we use nonequilibrium molecular dynamics simulations and stochastic thermodynamics to investigate energy transport in a well-studied nanoscale system—a molecular junction—subjected to a time-periodic temperature gradient. The primary observation is that molecular junctions can exhibit heat transport hysteresis, a phenomenon in which the heat flux through a system depends not only on the instantaneous value of a time-dependent temperature bias but also on the temporal history of that bias. The presented findings illustrate that molecular junctions can exhibit the specific memory effect—heat transport hysteresis—that is essential for the design of thermal neuromorphic computers. This work elucidates a potential pathway toward the realization of such devices.

Reimagining falls prevention with insights from systems mapping on the use of millimetre-wave radar for remote health monitoring

Abstract Falls constitute a significant public health concern, demanding innovative solutions that transcend traditional methodologies. Current falls practice focuses on reactive post-fall assessment and management rather than proactive prevention and mitigation. We propose that millimetre-wave radar technology for real-time, continuous falls risk screening at home may address the limitations of current falls practice. To investigate the feasibility of this solution, we interviewed five experts in physiotherapy, falls prevention among older adults, and comprehensive geriatric assessment to identify the current state of play and potential for changes to falls practice. We applied a novel technique, systems mapping, to visually illustrate and analyse the interactions between components of current and proposed systems for addressing falls and constructed two conceptual maps: First, the current system was mapped by asking experts about the causal relationships between 15 system components. Second, to examine the feasibility of the proposed system, the components related to falls risk screening were replaced by radar-based home monitoring and experts were asked to re-evaluate the causal relationships between system components. Next, four scenarios (no fear of falling, no mobility limitation, maximising screening in the current system, maximising radar-based screening) were applied using the maps. Experts identified mobility deterioration and previous falls as key indicators of future falls, noting that increased screening in current practice could reduce risks but increase healthcare professionals’ workloads. Experts were positive about radar-based wireless home monitoring, believing it could reduce fall risks whilst reducing all fall-related costs. These findings suggest that, according to experts, millimetre-wave radar can be an effective solution in advancing falls prevention.

Nonlinear response from linear oscillators: Gas phase 2D action spectroscopy

There have been rapid developments in new spectroscopic methods to collect coherent multidimensional optical spectra using incoherent action-detection schemes such as fluorescence or photocurrents. Recently, we demonstrated the acquisition of two-dimensional infrared (2D IR) spectra of molecular ions cryogenically cooled in the gas phase measured from photodissociation of a weakly bound N2 “tag” molecule. Important differences exist between traditional multidimensional spectra and their action-based counterparts, which are now just being fully realized. Here, we apply standard nonlinear response theory in the pure-dephasing limit to model the cryogenic ion 2D IR spectra of the complex fac-Re(CO)3(CH3CN)3+ in the carbonyl stretch region. The simulated spectra show overall good agreement with the experiment and provide key insights into some of the unique characteristics of action-based 2D spectra. Notably, cryogenic ion 2D IR spectra only display bleaching features and inherent cross peaks between all excited vibrational modes. Action-based 2D IR spectra, therefore, can be measured even in the absence of anharmonicity and anharmonic coupling between modes. While the response from any single mode can be fully modeled from parameters measured in the linear spectrum, under multi-mode excitation both diagonal and cross-peak intensities also depend on the relative dipole moment orientations of other vibrational modes on the same molecule, a quantity not available from linear spectra and often not easily obtained in traditional nonlinear spectroscopy.

Lubrication state identification of vibration time-frequency characteristics based on CWT and CNN

Anomalous expansion of interatomic distance in liquid Al–Zn alloy during cooling

The structural and physical/chemical properties of metallic materials are closely linked to the composition and configuration of their molten state. In this study, the evolution of the local structure in Al–Zn alloys with varying compositions during the quenching process was investigated using on-the-fly machine learning force field (MLFF) simulations based on ab initio molecular dynamics. The results indicate that the first coordination shell of the Al–Zn alloy melt undergoes an anomalous expansion within a specific temperature range, which deviates from the previously reported linear negative expansion, such as metallic Al, Zn, and Sn. The temperature interval of the anomalous expansion decreases with increasing concentration of Zn. Local structural changes, including the abnormal increase in coordination number and the emergence of a shoulder in the second peak of the bond angle distribution function, further confirmed the presence of the anomalous expansion. In addition, the slope of the energy–temperature relationship and the activation energy of diffusion change upon temperature decrease, which suggests that this phenomenon is associated with atomic diffusion driven by thermal excitation. The anomalous expansion is also confirmed in a larger system (over 104 atoms) by MLFF simulations with first-principles accuracy. We found that an increase in cluster radius induces a decrease in pressure due to the interfacial energy of the cluster at ∼1200–1100 K for Al–38Zn melts, which in turn leads to a looser arrangement of the atoms. This study provides valuable insights into the mechanisms governing atomic structure evolution and liquid–liquid transitions.

Correction: Horse handlers’ knowledge, attitudes, and perceptions of African horse sickness in South-West, Nigeria

Circumventing problems introduced by matrix asymmetry in collocation calculations of vibrational spectra by exploiting near symmetry

Collocation is an enticing alternative to variational methods for solving the vibrational Schrödinger equation. It makes it possible to use a general potential without requiring integrals and quadrature. An important disadvantage of collocation is the need to work with nonsymmetric matrices. Eigenvalues of a large matrix are best computed with an iterative method, but iterative eigensolvers are much more efficient for symmetric matrices. Heretofore, it has been costly to use collocation when the basis set and Hamiltonian matrix are large. We demonstrate that it is possible to systematically make the collocation matrix whose eigenvalues one must compute more and more symmetric and propose an efficient iterative eigensolver for a nearly symmetric matrix. Little is known about exploiting near symmetry. We use a combination of filter diagonalization and an iterative linear solver powered by a three-term recursion relation. We test the ideas with a 6-D Hamiltonian and show that accurate energies are obtained despite the asymmetry.

Robust zero-watermarking for color images using hybrid deep learning models and encryption

Abstract Reliable zero-watermarking is a distortion-free approach to copyright protection, which has been a primary focus of digital watermarking research. Traditional zero-watermarking techniques often struggle to maintain resilience against geometric and signal processing attacks while ensuring high security and imperceptibility. Many existing methods fail to extract stable and distinguishable features, making them vulnerable to image distortions such as compression, filtering, and geometric transformations. This paper presents a robust zero-watermarking technique for color images, combining Local Binary Patterns (LBP) with deep features extracted from the CONV5-4 layer of the VGG19 neural network to overcome these limitations. Frequent domain transformations, utilizing the Discrete Wavelet Transform (DWT) and Discrete Cosine Transform (DCT), enhance feature representation and improve resilience. Furthermore, a chaotic encryption scheme based on the Lorenz system and the Logistic map is used to scramble the feature matrix and watermark, thereby ensuring increased security. The zero watermark is generated through an XOR operation, facilitating imperceptible and secure ownership verification. Experimental results show that the proposed method is highly resilient to various attacks, including scaling, noise, filtering, compression, and rotation. The extracted watermark maintains a low Bit Error Rate (BER) and a high Normalized Cross-Correlation (NCC). At the same time, the Peak Signal-to-Noise Ratio (PSNR) of attacked images remains optimal. Specifically, the BER values of the extracted watermarks were below 0.0022, and the NCC values were above 0.9959. In contrast, the average PSNR values of the attacked images reached 34.0692 dB, demonstrating the method’s superior robustness and visual quality. Compared to existing zero-watermarking algorithms, the proposed method shows superior robustness and security, making it highly effective for multimedia copyright protection.

Transport mechanism of fluorosulfonylamide-based molten alkali metal salts—Intermediate temperature ionic liquids

Molecular dynamics (MD) simulations in this study elucidated the transport mechanism of a series of intermediate temperature ionic liquids: molten MFSA, MFTA, and MTFSA, where M = (Li, Na, K, Rb, and Cs), FSA = bis(fluorosulfonyl)amide, FTA = fluorosulfonyl(trifluoromethylsulfonyl)amide, and TFSA = bis(trifluoromethylsulfonyl)amide. The following two peculiarities had been experimentally observed: (i) the electrical conductivity, σ, of Li-systems is extremely lower than that of the other alkali metal counterparts and (ii) the Nernst–Einstein conductivity, σNE, derived from the self-diffusion coefficients, D+ and D−, of LiFSA and LiFTA is lower than the real conductivity, σ > σNE, which is usually the other way around. Hypothetical MD simulations made by increasing the size or decreasing the valence of Li+ revealed that the strong interaction between neighboring cation and anion caused by the high surface charge density on Li+ is responsible for both (i) and (ii). Theoretical consequences derived from the momentum conservation and the separation of σ into its components in terms of velocity correlation coefficients proved that, in addition to these features, the significant mass difference between a cation and anion for the Li-systems leads to (iii) the almost exclusive contribution of Li+ to σ and (iv) a positive contribution of the Li+–Li+ cross correlation, which is negative for other systems. Hypothetical simulations at high temperatures, at which the anions actually decompose, suggested that features (i), (ii), and (iv) stem from the “intermediate” temperature range at which these salts are fluid.

Tolerant integrated reciprocity sustains cooperation in a noisy environment

Isomerization-assisted proton transfers in MeOH-(H2O)2H+

We carried out Path Integral Molecular Dynamics simulations that describe the microscopic properties of two isomerization processes taking place in the MeOH(H2O)2H+ trimer, at T = 50 K. In particular, we focused attention on the free energies associated with the exchange of the connective/dangling characteristics of a pair of protons located at key subunits of the trimer. In one of the processes, the isomerization produces a modification in the topology of the cluster’s connectivity pattern, from branched-like to chain-like motifs. In contrast, along the other transformation, reactant and product states are both chain-like and equivalent. Changes in associated free energies were computed following reversible paths described in terms of order parameters involving angular degrees of freedom. As a common feature, along both isomerizations, we registered concomitant migrations of the excess proton. The strongly confining, single-well characteristics of the potential energy surface along the asymmetric stretch coordinate promote compact, ∼0.35 Å long structures for the isomorphic polymer associated with the itinerant proton, which persist along the complete isomerization path. These observations suggest the absence of tunneling contributions to the resulting mechanisms that control the proton transfer process. Estimates for the corresponding isomerization rates are also computed.