Controlled Formation of Conduction Channels in Memristive Devices Observed by X-ray Multimodal Imaging.

Neuromorphic computing provides a means for achieving faster and more energy efficient computations than conventional digital computers for artificial intelligence (AI). However, its current accuracy is generally less than the dominant software-based AI. The key to improving accuracy is to reduce the intrinsic randomness of memristive devices, emulating synapses in the brain for neuromorphic computing. Here using a planar device as a model system, the controlled formation of conduction channels is achieved with high oxygen vacancy concentrations through the design of sharp protrusions in the electrode gap, as observed by X-ray multimodal imaging of both oxygen stoichiometry and crystallinity. Classical molecular dynamics simulations confirm that the controlled formation of conduction channels arises from confinement of the electric field, yielding a reproducible spatial distribution of oxygen vacancies across switching cycles. This work demonstrates an effective route to control the otherwise random electroforming process by electrode design, facilitating the development of more accurate memristive devices for neuromorphic computing.

Near-Quantitative Predictions of the First-Shell Coordination Structure of Hydrated First-Row Transition Metal Ions Using K-Edge X-ray Absorption Near-Edge Spectroscopy.

The solvation structure of transition metal ions is important for applications in geochemistry, biochemistry, energy storage, and environmental chemistry. We study the X-ray absorption pre-edge and near-edge spectra at the K-edge of a nearly complete series of hydrated first-row transition metal ions with d orbital occupancy from d2 to d10. We optimize all of the structures at the density functional theory (DFT) level with explicit solvation and then compute the pre-edge X-ray absorption spectra with time-dependent DFT (TDDFT) and restricted active space second-order perturbation theory (RASPT2). TDDFT provides accurate results for spectra that are dominated by single excitations, while RASPT2 correctly distinguishes between singly and doubly excited states with quantitative accuracy compared with experiment. We analyze the pre-edge features for each metal ion to reveal the impact of the variations in d orbital occupancy on the first-shell coordination environment. We also report the lowest-energy ligand field d-d transitions using complete active space second-order perturbation theory.

Investigations of Bis(alkylthiocarbamato)copper Linkage Isomers.

Linkage isomers are coordination compounds with the same composition but different donor atoms, resulting in distinct physical and electronic structures. A pair of linkage isomers, CuL555 and CuL465, derived from phenylglyoxal bis(ethylthiocarbamate) were synthesized, isolated, and characterized by structural, electrochemical, and spectroscopic methods. The isomers are stable in solution under ambient conditions, but CuL465 converts to CuL555 in acid, consistent with quantum-chemical calculations. The complexes were screened against a lung adenocarcinoma cell line (A549) and a nonmalignant lung fibroblast cell line (IMR-90) to evaluate the antiproliferation activity. CuL555 and CuL465 possessed EC50 values of 0.113 ± 0.030 and 0.115 ± 0.038 µM for A549 and 1.87 ± 0.29 and 0.77 ± 0.22 µM for IMR-90, respectively.

Uptake of N2O5 by aqueous aerosol unveiled using chemically accurate many-body potentials.

The reactive uptake of N2O5 to aqueous aerosol is a major loss channel for nitrogen oxides in the troposphere. Despite its importance, a quantitative picture of the uptake mechanism is missing. Here we use molecular dynamics simulations with a data-driven many-body model of coupled-cluster accuracy to quantify thermodynamics and kinetics of solvation and adsorption of N2O5 in water. The free energy profile highlights that N2O5 is selectively adsorbed to the liquid-vapor interface and weakly solvated. Accommodation into bulk water occurs slowly, competing with evaporation upon adsorption from gas phase. Leveraging the quantitative accuracy of the model, we parameterize and solve a reaction-diffusion equation to determine hydrolysis rates consistent with experimental observations. We find a short reaction-diffusion length, indicating that the uptake is dominated by interfacial features. The parameters deduced here, including solubility, accommodation coefficient, and hydrolysis rate, afford a foundation for which to consider the reactive loss of N2O5 in more complex solutions.

Learning intermolecular forces at liquid-vapor interfaces.

By adopting a perspective informed by contemporary liquid-state theory, we consider how to train an artificial neural network potential to describe inhomogeneous, disordered systems. We find that neural network potentials based on local representations of atomic environments are capable of describing some properties of liquid-vapor interfaces but typically fail for properties that depend on unbalanced long-ranged interactions that build up in the presence of broken translation symmetry. These same interactions cancel in the translationally invariant bulk, allowing local neural network potentials to describe bulk properties correctly. By incorporating explicit models of the slowly varying long-ranged interactions and training neural networks only on the short-ranged components, we can arrive at potentials that robustly recover interfacial properties. We find that local neural network models can sometimes approximate a local molecular field potential to correct for the truncated interactions, but this behavior is variable and hard to learn. Generally, we find that models with explicit electrostatics are easier to train and have higher accuracy. We demonstrate this perspective in a simple model of an asymmetric dipolar fluid, where the exact long-ranged interaction is known, and in an ab initio water model, where it is approximated.

Abbreviated MRI in patients with suspected acute appendicitis in emergency: a prospective study.

PURPOSE: To determine the diagnostic performance of an abbreviated non-contrast MRI protocol in diagnosing acute appendicitis. METHODS: Prospectively, a total of 67 consenting consecutive patients with clinical suspicion of acute appendicitis (Alvarado score ≥ 5) were evaluated with an abbreviated three-sequence non-contrast MRI protocol (axial T2WI, coronal T2WI, axial DWI) at a single tertiary care center. MRI was interpreted by two radiologists blinded to the clinical details, other investigations, and outcome of the patients. Diagnostic performance of MRI was determined using either histopathological examination (HPE) results as the reference standard in surgical cases (n = 39), or final clinical diagnosis at discharge and 3-months follow-up in non-operatively managed cases (n = 28). RESULTS: Sixty-seven patients comprising 42 males, 25 females including 1 pregnant patient were enrolled (median age 24 years; age range 6-70 years). The median acquisition duration of the MRI protocol was 12.5 min. In the analysis of the complete cohort including both surgical and non-operatively managed cases (n = 67), MRI showed sensitivity of 93.3% (95% CI 81.7-98.6%), specificity of 86.4% (95% CI 65.1-97.1%), and diagnostic accuracy of 91.0% (95% CI 81.5-96.6%) (p < 0.001). In the subset of surgical cases with HPE as the reference standard (n = 39), MRI showed sensitivity of 97.1% (95% CI 84.7-99.9%), specificity of 100% (95% CI 47.8-100%), and diagnostic accuracy of 98% (95% CI 87.5-100%) (p < 0.001). CONCLUSION: MRI may be performed to diagnose acute appendicitis or alternative causes of right iliac fossa pain. An abbreviated MRI protocol consisting of only three sequences without IV contrast, patient preparation, or antiperistaltic agents could shorten the examination duration while retaining diagnostic accuracy.

Reactive uptake of N2O5 by atmospheric aerosol is dominated by interfacial processes.

Nitrogen oxides are removed from the troposphere through the reactive uptake of N2O5 into aqueous aerosol. This process is thought to occur within the bulk of an aerosol, through solvation and subsequent hydrolysis. However, this perspective is difficult to reconcile with field measurements and cannot be verified directly because of the fast reaction kinetics of N2O5 Here, we use molecular simulations, including reactive potentials and importance sampling, to study the uptake of N2O5 into an aqueous aerosol. Rather than being mediated by the bulk, uptake is dominated by interfacial processes due to facile hydrolysis at the liquid-vapor interface and competitive reevaporation. With this molecular information, we propose an alternative interfacial reactive uptake model consistent with existing experimental observations.

Quantifying the hydration structure of sodium and potassium ions: taking additional steps on Jacob's Ladder.

The ability to reproduce the experimental structure of water around the sodium and potassium ions is a key test of the quality of interaction potentials due to the central importance of these ions in a wide range of important phenomena. Here, we simulate the Na+ and K+ ions in bulk water using three density functional theory functionals: (1) the generalized gradient approximation (GGA) based dispersion corrected revised Perdew, Burke, and Ernzerhof functional (revPBE-D3) (2) the recently developed strongly constrained and appropriately normed (SCAN) functional (3) the random phase approximation (RPA) functional for potassium. We compare with experimental X-ray diffraction (XRD) and X-ray absorption fine structure (EXAFS) measurements to demonstrate that SCAN accurately reproduces key structural details of the hydration structure around the sodium and potassium cations, whereas revPBE-D3 fails to do so. However, we show that SCAN provides a worse description of pure water in comparison with revPBE-D3. RPA also shows an improvement for K+, but slow convergence prevents rigorous comparison. Finally, we analyse cluster energetics to show SCAN and RPA have smaller fluctuations of the mean error of ion-water cluster binding energies compared with revPBE-D3.

Supersaturated calcium carbonate solutions are classical.

Mechanisms of CaCO3 nucleation from solutions that depend on multistage pathways and the existence of species far more complex than simple ions or ion pairs have recently been proposed. Herein, we provide a tightly coupled theoretical and experimental study on the pathways that precede the initial stages of CaCO3 nucleation. Starting from molecular simulations, we succeed in correctly predicting bulk thermodynamic quantities and experimental data, including equilibrium constants, titration curves, and detailed x-ray absorption spectra taken from the supersaturated CaCO3 solutions. The picture that emerges is in complete agreement with classical views of cluster populations in which ions and ion pairs dominate, with the concomitant free energy landscapes following classical nucleation theory.

Mass density fluctuations in quantum and classical descriptions of liquid water.

First principles molecular dynamics simulation protocol is established using revised functional of Perdew-Burke-Ernzerhof (revPBE) in conjunction with Grimme's third generation of dispersion (D3) correction to describe the properties of water at ambient conditions. This study also demonstrates the consistency of the structure of water across both isobaric (NpT) and isothermal (NVT) ensembles. Going beyond the standard structural benchmarks for liquid water, we compute properties that are connected to both local structure and mass density fluctuations that are related to concepts of solvation and hydrophobicity. We directly compare our revPBE results to the Becke-Lee-Yang-Parr (BLYP) plus Grimme dispersion corrections (D2) and both the empirical fixed charged model (SPC/E) and many body interaction potential model (MB-pol) to further our understanding of how the computed properties herein depend on the form of the interaction potential.

Molecular dynamics simulations predict an accelerated dissociation of H2CO3 at the air-water interface.

The dissociation and decomposition reactions of carbonic acid (H2CO3) in bulk water have been thoroughly studied, but little is known about its reactivity at the air-water interface. Herein, we investigate the dissociation reaction of H2CO3 at the air-water interface using ab initio molecular dynamics and metadynamics. Our results indicate that H2CO3 (pKa = 3.45) dissociates faster at the water surface than in bulk water, in contrast to recent experiments and simulations which have shown that HNO3 (pKa = -1.3) has a lower propensity to dissociate at the water surface than in bulk water. We find that the water surface allows for a more structured solvation environment around H2CO3 than in bulk water, which contributes to a decrease in the dissociation energy barrier via a stabilization of the transition state relative to the undissociated acid. Given its decreased kinetic stability at the air-water interface, H2CO3 may play an important role in the acidification of atmospheric aerosols and water droplets.

The role of hydrogen bonding in the decomposition of H2CO3 in water: mechanistic insights from ab initio metadynamics studies of aqueous clusters.

Both concerted and stepwise mechanisms have been proposed for the decomposition of H2CO3 in bulk water based on electronic structure and ab initio molecular dynamics calculations. To consistently determine which, if any, mechanism predominates in bulk water, we performed ab initio metadynamics simulations of the decomposition of H2CO3 in water clusters of increasing size. We found that, in the small clusters (containing six and nine water molecules), the decomposition occurs according to a concerted proton shuttle mechanism via a cyclic transition state, whereas, in the larger clusters (containing 20 and 45 water molecules), the decomposition occurs according to a two-step mechanism via a solvent-separated HCO3⁻/H3O⁺ ion pair intermediate. Due to the additional water molecules in the larger clusters, the dissociation of H2CO3 into the metastable solvent-separated ion pair was found to be energetically favorable, thereby preventing the formation of the cyclic transition state and committing the decomposition to the sequential route. An analysis of the solvation environment around the H2CO3 molecule in the various clusters revealed that the transition from the concerted mechanism to the stepwise mechanism precisely hinges upon the number of water molecules hydrogen bonded to the H3O⁺ intermediate, which changes as the size of the cluster increases. The larger clusters contain a sufficient number of water molecules to fully solvate the H3O⁺ intermediate, indicating that they can provide a bulk-like environment for this reaction. Therefore, these results strongly demonstrate that the decomposition of H2CO3 in bulk water occurs via the stepwise mechanism.

Mechanistic insights into the dissociation and decomposition of carbonic acid in water via the hydroxide route: an ab initio metadynamics study.

The dissociation and decomposition of carbonic acid (H2CO3) in water are important reactions in the pH regulation in blood, CO2 transport in biological systems, and the global carbon cycle. H2CO3 is known to have three conformers [cis-cis (CC), cis-trans (CT), and trans-trans (TT)], but their individual reaction dynamics in water has not been probed experimentally. In this paper, we have investigated the energetics and mechanisms of the conformational changes, dissociation (H2CO3 -->/<-- HCO3(-) + H(+)), and decomposition via the hydroxide route (HCO3(-) --> CO2+OH(-)) of all three conformers of H2CO3 in water using Car-Parrinello molecular dynamics (CPMD) in conjunction with metadynamics. It was found that, unlike in the gas phase, the interconversion between the various conformers occurs via two different pathways, one involving a change in one of the two dihedral angles (O=C-O-H) and the other a proton transfer through a hydrogen-bond wire. The free energy barriers/changes for the various conformational changes via the first pathway were calculated and contrasted with the previously calculated values for the gas phase. The CT and TT conformers were found to undergo decomposition in water via a two-step process: first, the dissociation and then the decomposition of HCO3(-) into CO2 and OH(-). The CC conformer does not directly decompose but first undergoes a conformational change to CT or TT prior to decomposition. This is in contrast with the concerted mechanism proposed for the gas phase, which involves a dehydroxylation of one of the OH groups and a simultaneous deprotonation of the other OH group to yield CO2 and H2O. The dissociation in water was seen to involve the repeated formation and breakage of a hydrogen-bond wire with neighboring water molecules, whereas the decomposition is initiated by the diffusion of H(+) away from HCO3(-); this decomposition mechanism differs from that proposed for the water route dehydration (HCO3(-) + H3O(+) --> CO2 + H2O), which involves the participation of a nearbyH3O(+) ion.Our calculated pKa values and decomposition free energy barriers for the CT and TT conformers are consistent with the overall experimental values of 3.45 and 22.28 kcal/mol, respectively, suggesting that the dynamics of the various conformers should be taken into account for a better understanding of aqueous H2CO3 chemistry.