Repulsive Term Research Articles

The Many-Body Expansion for Aqueous Systems Revisited: IV. Stabilization of Halide-Anion Pairs in Small Water Clusters.

We report the structures, energetics, many-body effects, and vibrational spectra of water clusters stabilizing pairs of halide-anions, X-(H2O)kY- (k = 2-6, X/Y = F, Cl, Br, I) as well as their stability in the gas phase relative to fragmentation. We find that these metastable cluster structures mimicking the solvent-separated ion pair (SSIP) configurations in aqueous solutions are less stable relative to fragmentation into smaller ionic aqueous clusters containing a single halide anion. The many-body expansion (MBE) at these geometries was found to converge at the 4-body term, which is, however, significant, amounting to >20% of the total binding energy in several instances. The binding motif of these ion pair aqueous clusters starts as networks in which the water molecules form a "bridge" between the two halide-anions. As the cluster grows, these structures become destabilized by a more repulsive 3-body term for distances R(O-O) < 3.45 Å with respect to networks in which water molecules move outside the bridge, solvating the other side of the anion.

Repulsion, chaos, and equilibrium in mixture models

Abstract Mixture models are commonly used in applications with heterogeneity and overdispersion in the population, as they allow the identification of subpopulations. In the Bayesian framework, this entails the specification of suitable prior distributions for the weights and locations of the mixture. Despite their popularity, the flexibility of these models often does not translate into the interpretability of the clusters. To overcome this issue, repulsive mixture models have been recently proposed. The basic idea is to include a repulsive term in the distribution of the atoms of the mixture, favouring mixture locations far apart. This approach induces well-separated clusters, aiding the interpretation of the results. However, these models are usually not easy to handle due to unknown normalizing constants. We exploit results from equilibrium statistical mechanics, where the molecular chaos hypothesis implies that nearby particles spread out over time. In particular, we exploit the connection between random matrix theory and statistical mechanics and propose a novel class of repulsive prior distributions based on Gibbs measures associated with joint distributions of eigenvalues of random matrices. The proposed framework greatly simplifies computations thanks to the availability of the normalizing constant in closed form. We investigate the theoretical properties and clustering performance of the proposed distributions.

Phase Diagrams of a Relativistic Self-Interacting Boson System

Within the Canonical Ensemble, we investigate a system of interacting relativistic bosons at finite temperatures and finite isospin densities in a mean-field approach. The mean field contains both attractive and repulsive terms. Temperature and isospin density dependences of thermodynamic quantities are obtained. It is shown that, in the case of attraction between particles in a bosonic system, a liquid-gas phase transition develops against the background of the Bose–Einstein condensate. The corresponding phase diagrams are given. We explain the reasons for why the presence of a Bose condensate significantly increases the critical temperature of the liquid-gas phase transition compared to that obtained for the same system within the framework of Boltzmann statistics. Our results may have implications for the interpretation of experimental data, in particular, how sensitive the critical point of the mixed phase is to the presence of the Bose–Einstein condensate.

Completely Multipolar Model as a General Framework for Many-Body Interactions as Illustrated for Water.

We introduce a general framework for many-body force fields, the Completely Multipolar Model (CMM), that utilizes multipolar electrical moments modulated by exponential decay of electron density as a common functional form for all terms of an energy decomposition analysis of intermolecular interactions. With this common functional form, the CMM model establishes well-formulated damped tensors that reach the correct asymptotes at both long- and short-range while formally ensuring no short-range catastrophes. CMM describes the separable EDA terms of dispersion, exchange polarization, and Pauli repulsion with short-ranged anisotropy, polarization as intramolecular charge fluctuations and induced dipoles, while charge transfer describes explicit movement of charge between molecules, and naturally describes many-body charge transfer by coupling into the polarization equations. We also utilize a new one-body potential that accounts for intramolecular polarization by including an electric field-dependent correction to the Morse potential to ensure that CMM reproduces all physically relevant monomer properties including the dipole moment, molecular polarizability, and dipole and polarizability derivatives. The quality of CMM is illustrated through agreement of individual terms of the EDA and excellent extrapolation to energies and geometries of an extensive validation set of water cluster data.

Enhancing YOLO for occluded vehicle detection with grouped orthogonal attention and dense object repulsion

In real-life complex traffic environments, vehicles are often occluded by extraneous background objects and other vehicles, leading to severe degradation of object detector performance. To address this issue, we propose a method named YOLO-OVD (YOLO for occluded vehicle detection) and a dataset for effectively handling vehicle occlusion in various scenarios. To highlight the model attention in unobstructed region of vehicles, we design a novel grouped orthogonal attention (GOA) module to achieve maximum information extraction between channels. We utilize grouping and channel shuffling to address the initialization and computational issues of original orthogonal filters, followed by spatial attention for enhancing spatial features in vehicle-visible regions. We introduce a CIoU-based repulsion term into the loss function to augment the network’s localization accuracy in scenarios involving densely packed vehicles. Moreover, we explore the effect of the knowledge-based Laplacian Pyramid on the OVD performance, which contributes to fast convergence in training and ensures more detailed and comprehensive feature retention. We conduct extensive experiments on the established occluded vehicle detection dataset, which demonstrates that the proposed YOLO-OVD model significantly outperforms 14 representative object detectors. Notably, it achieves improvements of 4.7% in Precision, 3.6% in AP@0.5, and 1.9% in AP@0.5:0.95 compared to the YOLOv5 baseline.

Molecular Hydrogen Acts as a Hydrogen Bond Proton Acceptor: From Protonated Betaine Tagging to the Weakest Hydrogen Bond.

In an attempt to gain further insights into the intermolecular interactions implied by Rizzo's group's cautionary tale related to molecular tagging in infrared multiple photon dissociation (IRMPD) spectroscopy with molecular messengers [Masson, A. . J. Chem. Phys. 2015, 143, 104313], in the present study, we provide an in-depth analysis of the noncovalent interaction between the molecular hydrogen and protonated betaine molecule in the gas phase. We aim to shed some new light on the fundamental issues concerning the wide diapason of hydrogen-bonding-type intermolecular interactions, with a wide variety of proton acceptors. We demonstrate that in the course of tagging the protonated betaine with molecular hydrogen from the OH group side, it is the σ bond of molecular hydrogen that plays the role of hydrogen-bonding proton acceptor. The tagging thus induces a small yet significant red shift of the protonated betaine O-H stretching mode. We investigate the performance of a wide range of density functional theory (DFT) functionals for the calculation of anharmonic vibrational frequency shifts of the studied system, which are essential for the correct interpretation of the experimental IRMPD data. For an accurate prediction of the OH stretching frequency shifts, specifically designed functionals such as Handy's group HCTH/407 should be applied. The empirical dispersion correction enhances the systematic overestimation of the anharmonic frequency shift, characteristic of the most widely used DFT functionals. Combining the full-wave function approach with the charge field perturbation and natural bond orbital (NBO) deletion analyses, we demonstrate that the frequency shift in the OH-tagged structure is governed by the σHH → σ*OH intermolecular charge transfer. This interaction stabilizes the OH-tagged dimer as well, in contrast to the dipole-quadrupole electrostatic interaction energy term. Topological analysis of the electron density reveals the presence of an intermolecular bond critical point with a positive value of the density Laplacian very close to the lower limit for hydrogen bonds. NCI analyses demonstrate that the OH···H2 interaction is weaker than the intramolecular CH···O one within the protonated betaine molecule, with the through of reduced density gradient appearing at less negative sign(λ2)·ρ values. Analyzing the O-H stretching vibrational potential with the second-generation absolutely localized molecular orbitals energy decomposition analysis (ALMO-EDA 2) revealed that in the case of betaineH(+) tagged from the OH group side, the permanent electrostatics (ΔEelec), polarization (ΔEpol), and charge-transfer (ΔEct) contributions to the total intermolecular interaction energy contribute favorably to the weak hydrogen bond formation and to the red shift of the fundamental O-H stretching frequency, the ΔEct contribution being the most significant in the last context. The Pauli repulsion term, on the other hand, favors an O-H stretching frequency blue shift as a consequence of the vibrational confinement effects.

Structural transitions, octahedral rotations, and electronic properties of A3Ni2O7 rare-earth nickelates under high pressure

Motivated by the recent observation of superconductivity with Tc ~ 80 K in pressurized La3Ni2O71, we explore the structural and electronic properties of A3Ni2O7 bilayer nickelates (A = La-Lu, Y, Sc) as a function of pressure (0–150 GPa) from first principles including a Coulomb repulsion term. At ~ 20 GPa, we observe an orthorhombic-to-tetragonal transition in La3Ni2O7 at variance with x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram that establishes chemical and external pressure as distinct and counteracting control parameters. We find unexpected correlations between Tc and the in-plane Ni-O-Ni bond angles for La3Ni2O7. Moreover, two structural phases with significant c+ octahedral rotations and in-plane bond disproportionations are uncovered for A = Nd-Lu, Y, Sc that exhibit a pressure-driven electronic reconstruction in the Ni eg manifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb3Ni2O7 as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism.

Three-Center Tight-Binding Together with Multipolar Auxiliary Functions.

We present an ab initio tight-binding method that allows to improve on the effective potential and minimal basis approximations employed in semiempirical calculations. Three-center expansions are used to evaluate the zeroth-order Hamiltonian matrix elements and repulsive energy terms in the spirit of the Horsfield method. Self-consistency is handled by expanding atomic orbital products in an auxiliary basis following the work of Giese and York, combined with a two-center expansion of the exchange-correlation kernels. Together with nonminimal main basis sets (double-ζ plus polarization), we show that the resulting method trades a modest amount of accuracy for a significant gain in speed, compared to that of numerical atomic orbital density functional theory, in calculations on small molecules, bulk compounds, and metal nanoclusters.

Gravitational repulsion in an expanding ball of dust

In general relativity, there is a velocity dependent term in the gravitational acceleration of a test particle for an observer at infinity. Depending on the direction of motion and the speed, that term can be repulsive. We show that this is also the case in the Parametrized Post-Newtonian (PPN) formalism. We compute the magnitude of that repulsive term for an expanding sphere of dust observed at infinity, and find that it could mimic the effect of a cosmological constant. The time evolution of such an expanding ball of dust for an observer at infinity is calculated, and compared with the standard ΛCDM model. We find that the so-called coincidence problem does not exist for such a model as the energy density attributed to the expansion is always of the same order as the matter energy density.

Tuning the carrier localization, magnetic and thermoelectric properties of ultrathin ( LaNiO3−δ)1 /( LaAlO3)1 (001) superlattices by oxygen vacancies

Using a combination of density functional theory calculations with an on-site Coulomb repulsion term (DFT+U) and Boltzmann transport theory within the constant relaxation time approximation, we explore the effect of oxygen vacancies on the electronic, magnetic, and thermoelectric properties in ultrathin (LaNiO3−δ)1/(LaAlO3)1(001) superlattices (SLs). For the pristine SL (δ=0), an antiferromagnetic charge-disproportionated (AFM-CD) (d8L̲2)S=0(d8)S=1 phase is stabilized, irrespective of strain. At δ=0.125 and 0.25, the localization of electrons released from the oxygen defects in the NiO2 plane triggers a charge-disproportionation, leading to a ferrimagnetic insulator both at aSrTiO3 (tensile strain) and aLaSrAlO4 (compressive strain). At δ=0.5, an insulating phase emerges with alternating stripes of Ni2+ (high-spin) and Ni2+ (low-spin) and oxygen vacancies ordered along the [110] direction (S-AFM), irrespective of strain. This results in a robust n-type in-plane power factor of 24µW/K2 cm at aSTO and 14µW/K2 cm at aLSAO at 300 K (assuming relaxation time τ=4 fs). Additionally, the pristine and δ = 0.5 SLs are shown to be dynamically stable. This demonstrates the fine tunability of electronic, magnetic, and thermoelectric properties of ultrathin nickelate superlattices by oxygen vacancies. Published by the American Physical Society 2024

Influence of material characteristics on plant-based milk alternative properties

Plant-based milk alternatives are gaining increasing relevance in the food industry, but the influence of plant proteins and fibers on the specific product properties has not yet been explored. In this study, the influence of soy protein isolate and the oat fiber β-glucan on the emulsion and powder properties of plant milk alternatives is analyzed. The components exhibit different behavior at the oil-water interface after homogenization, thereby affecting droplet size distribution, dynamic viscosity and emulsion stability in terms of electrostatic repulsion. To determine the interactions of the components at the interface, the individual raw materials soy protein isolate and oat bran as well as mixtures of both were investigated. The measurements highlight variations between the fiber component and the plant protein source at the interface. The identified differences in emulsion characteristics also manifest in spray drying with different particle sizes and lipid encapsulation efficiencies.

General spherically symmetric solution of Cotton gravity

In this paper we present the general spherically symmetric static solution to the vacuum equations of Cotton gravity. The obtained metric solution reveals the presence of singularities at the photosphere of a spherical source, which probably obstruct the formation of the stellar Schwarzschild-radius black holes. The solution is characterized by two integration constants, whose values can be restricted by association with the Hubble horizon. We examine the diverse features of the solution, including the long-range modifications to Newton’s force through the incorporation of the velocity-squared repulsive term to model the dark energy.

A Cubic Equation of State with a Repulsion Term

A new cubic equation of state (EoS) was proposed to address the lack of repulsion terms in conventional cubic EoSs, with a molar critical compressibility factor Zc = 0.273 and suitable for most molecular fluids. Similar to Soave-Redlich-Kwong (SRK) EoS, its α-function is related to the fluids’ acentric factors by fitting with 44 molecular fluids. Differently, the new EoS can pass the α-consistency test, confirming the robustness across all temperatures. With only three parameters (ac, b, m) to be determined, it is as convenient as the SRK EoS in application. But it is more accurate - tested against experimental data - for both single and multi-component fluids than SRK EoS.

Gaussian representation of coarse-grained interactions of liquids: Theory, parametrization, and transferability.

Coarse-grained (CG) interactions determined via bottom-up methodologies can faithfully reproduce the structural correlations observed in fine-grained (atomistic resolution) systems, yet they can suffer from limited extensibility due to complex many-body correlations. As part of an ongoing effort to understand and improve the applicability of bottom-up CG models, we propose an alternative approach to address both accuracy and transferability. Our main idea draws from classical perturbation theory to partition the hard sphere repulsive term from effective CG interactions. We then introduce Gaussian basis functions corresponding to the system's characteristic length by linking these Gaussian sub-interactions to the local particle densities at each coordination shell. The remaining perturbative long-range interaction can be treated as a collective solvation interaction, which we show exhibits a Gaussian form derived from integral equation theories. By applying this numerical parametrization protocol to CG liquid systems, our microscopic theory elucidates the emergence of Gaussian interactions in common phenomenological CG models. To facilitate transferability for these reduced descriptions, we further infer equations of state to determine the sub-interaction parameter as a function of the system variables. The reduced models exhibit excellent transferability across the thermodynamic state points. Furthermore, we propose a new strategy to design the cross-interactions between distinct CG sites in liquid mixtures. This involves combining each Gaussian in the proper radial domain, yielding accurate CG potentials of mean force and structural correlations for multi-component systems. Overall, our findings establish a solid foundation for constructing transferable bottom-up CG models of liquids with enhanced extensibility.

A note on some non-local variational problems

We study two non-local variational problems that are characterized by the presence of a Riesz-like repulsive term that competes with an attractive term. The first functional is defined on the subsets of the Euclidean space and has the fractional perimeter as an attractive term. The second functional instead is defined on non-negative integrable and uniformly bounded densities and contains an attractive term of positive-power type. For both of the functionals, we prove that balls are the unique minimizers in the appropriate volume constraint range, generalizing the results already present in the literature for more specific energies.

Effect of temperature on the appearance of flocculated asphaltene in an unperturbed unstable American shale oil: Insights from a colloidal modeling

This work aimed to study the onset of flocculation of asphaltenes using optical microscopy and to evaluate the effect of temperature on the destabilization of an unstable undisturbed American shale oil. After magnetic stirring for 24 h, the flocculated asphaltene was redispersed. Increasing the temperature to 40 °C led to asphaltenes flocculating. After heating, the sample was cooled back to room temperature. We calculated the precipitate volume from microscopy data and found that after cooling the system had a larger precipitate volume than in the previously studied scenarios. When comparing the volumes of precipitates before and after cooling, it is possible to observe that the volume of smaller particles does not vary significantly in each class, but the volume of larger particles increases with cooling, indicating an irreversible phenomenon. Statistical analysis of the optical microscopy results allowed a detailed analysis of the phenomenon. To study the colloidal effect, a theoretical analysis was performed based on Hamaker theory and De Gennes scaling theory, presenting compatible results when the parameters of the repulsive term were adjusted. When the parameters of both the attractive term and the repulsive term of the model were kept constant, greater stability was verified with the increase of temperature. Conversely, it was observed that the reduction of the parameter related to the length of the alkyl chains can explain the experimental macroscopic behavior of the shale oil here studied and also some different behaviors reported in the literature, i. e., increasing of flocculated volume with increasing temperature. This size reduction of the length of the alkyl chains can be attributed to a desolvation effect. Hence, the appropriate parameterization of the model showed great potential to mimic the behavior of increasing and also decreasing stability with the increase of temperature. The sets of parameters were related to reaction-limited aggregation (RLA) and diffusion-limited aggregation (DLA), which allowed to interconnect the discussion on experimental stability of asphaltenes, molecular simulation, and the theoretical discussion. Furthermore, the proper use of this theory seems to be a promising way to represent microscopic interactions in asphaltene systems, including previous results from the literature. This work presents (i) an experimental case that shows reversibility of asphaltene flocculation only with magnetic stirring; (ii) the effect of temperature on the stability of asphaltenes, and also (iii) brings a contribution with the volume of precipitate, making it possible to conclude on the irreversibility in the thermal cycle applied. In addition, the proper use of this modeling can predict the stability changes as a function of temperature, being a promising way to represent microscopic interactions in asphaltene systems, including previous results from the literature.

Cosmic voids and the kinetic analysis

We study structure formation in the late Universe within the Vlasov kinetic self-consistent field approach. Our work is principally focused on the use of the modified gravitational potential with a repulsive term of the cosmological constant, which is directly linked to observations that enable characterizations of the Hubble tension as the result of local and global flows. We formulate the criteria for the formation of the semi-periodic gravitating structures, along with the predictions of their quantitative scales associated with observable parameters. Our principal conclusion is that filament formation in the Local (late) Universe can proceed as a deterministic process that is distinct from the structures at larger scales that result from the essentially stochastic dynamics of density perturbations.

Interaction of one-dimensional quantum droplets with potential wells and barriers

We address static and dynamical properties of one-dimensional (1D) quantum droplets (QDs) under the action of local potentials in the form of narrow wells and barriers. The dynamics of QDs is governed by the 1D Gross–Pitaevskii equation including the mean-field cubic repulsive term and the beyond-mean-field attractive quadratic one. In the case of the well represented by the delta-function potential, three exact stable solutions are found for localized states pinned to the well. The Thomas–Fermi approximation for the well and the adiabatic approximation for the collision of the QD with the barrier are developed too. Collisions of incident QDs with the wells and barriers are analyzed in detail by means of systematic simulations. Outcomes, such as fission of the moving QD into transmitted, reflected, and trapped fragments, are identified in relevant parameter planes. In particular, a counter-intuitive effect of partial or full rebound of the incident QD from the potential well is studied in detail and qualitatively explained.

Gradient-based adaptive importance samplers

Importance sampling (IS) is a powerful Monte Carlo methodology for the approximation of intractable integrals, very often involving a target probability distribution. The performance of IS heavily depends on the appropriate selection of the proposal distributions where the samples are simulated from. In this paper, we propose an adaptive importance sampler, called GRAMIS, that iteratively improves the set of proposals. The algorithm exploits geometric information of the target to adapt the location and scale parameters of those proposals. Moreover, in order to allow for a cooperative adaptation, a repulsion term is introduced that favors a coordinated exploration of the state space. This translates into a more diverse exploration and a better approximation of the target via the mixture of proposals. Moreover, we provide a theoretical justification of the repulsion term. We show the good performance of GRAMIS in two problems where the target has a challenging shape and cannot be easily approximated by a standard uni-modal proposal.

High Chern numbers in a perovskite-derived dice lattice (LaXO3)3/(LaAlO3)3(111) with X = Ti, Mn and Co

The dice lattice, containing a stack of three triangular lattices, has been proposed to exhibit nontrivial flat bands with nonzero Chern numbers, but unlike the honeycomb lattice it is much less studied. By employing density-functional theory (DFT) calculations with an on-site Coulomb repulsion term, we explore systematically the electronic and topological properties of (LaXO3)3/(LaAlO3)3(111) superlattices with X = Ti, Mn and Co, where a LaAlO3 trilayer spacer confines the LaXO3 (LXO) dice lattice. In the absence of spin-orbit coupling (SOC) with symmetry constrained to P3, the ferromagnetic (FM) phase of the LXO(111) trilayers exhibits a half-metallic band structure with multiple Dirac crossings and coupled electron-hole pockets around the Fermi energy. Symmetry lowering induces a significant rearrangement of bands and triggers a metal-to-insulator transition. Inclusion of SOC leads to a substantial anomalous Hall conductivity (AHC) around the Fermi energy reaching values up to sim -3e^2/h for X = Mn and Co in P3 symmetry and both in- and out-of-plane magnetization directions in the first case and along [001] in the latter. The dice lattice emerges as a promising playground to realise nontrivial topological phases with high Chern numbers.