Multiatom Interactions Research Articles

Electrically enhanced adsorption efficiency of aluminum nitride nanotube for sulfate ion removal from water

Herein, the adsorption performance of sulfate ion in water on aluminum nitride nanotube(AlNNT) under the influence of an electric field was investigated using the density functional theory (DFT) calculation method. The model structure stability, adsorption energy, electronic and thermodynamic properties of sulfate ion adsorbed on the surface of AlNNT were studied. The calculation results indicate that sulfate ion reacts with multi-atoms on the surface of AlNNT, forming ionic bonds and undergoing chemical adsorption. As the electric field intensity increases, the adsorption energy and the transfer of electrons from sulfate ion to AlNNT increase, leading to a higher degree of hybridization of atomic orbitals and enhanced multi-atom interactions. Additionally, the thermodynamic data suggests that the adsorption between sulfate ion and AlNNT under electric field can occur spontaneously, the process of which is exothermic. The results of present study are expected to propose a novel method for separation and removal of sulfate pollutants from water.

Transferring chemical and energetic knowledge between molecular systems with machine learning

Predicting structural and energetic properties of a molecular system is one of the fundamental tasks in molecular simulations, and it has applications in chemistry, biology, and medicine. In the past decade, the advent of machine learning algorithms had an impact on molecular simulations for various tasks, including property prediction of atomistic systems. In this paper, we propose a novel methodology for transferring knowledge obtained from simple molecular systems to a more complex one, endowed with a significantly larger number of atoms and degrees of freedom. In particular, we focus on the classification of high and low free-energy conformations. Our approach relies on utilizing (i) a novel hypergraph representation of molecules, encoding all relevant information for characterizing multi-atom interactions for a given conformation, and (ii) novel message passing and pooling layers for processing and making free-energy predictions on such hypergraph-structured data. Despite the complexity of the problem, our results show a remarkable Area Under the Curve of 0.92 for transfer learning from tri-alanine to the deca-alanine system. Moreover, we show that the same transfer learning approach can also be used in an unsupervised way to group chemically related secondary structures of deca-alanine in clusters having similar free-energy values. Our study represents a proof of concept that reliable transfer learning models for molecular systems can be designed, paving the way to unexplored routes in prediction of structural and energetic properties of biologically relevant systems.

Combined Effects of Mn, C, and H on the Stacking Fault Energy in Austenitic Mn Steels

Manganese, carbon, and hydrogen are common alloying elements in austenitic Mn steels, which all influence the stacking fault energy (SFE), with Mn and H decreasing while C increasing it. The combined effects of Mn, C, and H on the SFE in austenitic Fe1−xMnx alloys, including pure Fe, Fe31Mn, Fe15Mn, Fe7Mn, and Fe3Mn, using first‐principles calculations, are investigated. The enhancing effect of C on the SFE is 29 mJ m−2 lower in Fe3Mn than in pure Fe, whereas it remains largely unchanged for low Mn concentrations. The noticeable combined Mn−C effect for Fe3Mn is mainly due to the formation of Mn−C−Mn trimer. Similarly, the boosting effect of H is weakened with increasing Mn content. As for the ternary Mn−C−H effect, it is found that it is generally stronger than that of binary effects. All the binary and ternary effects are much smaller than those of individual elements, indicating weak multiatom interactions of Mn, C, and H.

Microscopic theory of the second- and third-order elastic constants of cobalt in a thermodynamic module

The total potential energy of a crystal is presented as an expansion by irreducible interactions in clusters containing pairs of atomic triplets and quadruplets. The potential energy of the clusters in the adiabatic approximation is a function of vectors \(\vec r_{ik}\) connecting the centers of atoms in the clusters. Arguments (basic functions) affecting the potential energy of clusters are found using the model with allowance for the exchange symmetry of atoms and the irreducibility of the considered energies of doublet, triplet, and quadruplet atoms and interactions. This allows us to present arguments of the potential energy in the form of summed integral numbers (latticed sums) multiplied by a fixed value of the crystal unit cell parameter, and to set the numerical values of the potential energy arguments as a function of vectors \(\vec r_{ik}\). A potential corresponding to the thermodynamic additivity concept of crystal energy is selected as the model potential of pairwise interaction. Secondand third-order moduli of the rigidity of Co crystals with A1, A2, and HCP structures are calculated using this model of multiatom interactions.

Theoretical treatment of the interaction between two-level atoms and periodic waveguides.

Light transport in periodic waveguides coupled to two-level atoms is investigated theoretically. By using optical Bloch equations and a photonic modal formalism, we derive a convenient semi-analytical expression for calculating the scattering matrix of single atoms trapped in periodic waveguides. The expression that holds for both photonic and plasmonic waveguides represents a basic building block toward the study of collective effects arising from photon-mediated multi-atom interactions in periodic waveguides.

Second virial coefficient and mechanical moduli of metallic glasses

The relationship between the bulk, shear moduli and second virial coefficient of amorphous materials is derived according to their dependences with the radial distribution function. Lennard-Jones–Gaussian potential is used to investigate the relationship between second virial coefficient and temperature, where Lennard-Jones potential represents interactions with the nearest neighbor atoms, and Gaussian potential is responsible for the multi-atom interactions including the next nearest neighbor atoms and heterogeneous structures for a metallic glass. The results show that deep potential well formed by Gaussian potential causes a large second virial coefficient at low temperatures, which is very obvious for the larger fragility glasses. The quadratic form relationship of shear modulus and compositions is proposed, and confirmed by the experimental results of PdxNi100−x−20P20 alloy.

On collective effects in cavity quantum electrodynamics

We investigate the role of collective effects in the micromaser system as used in various studies of the physics of cavity electrodynamics. We focus our attention on the effect on large-time correlations due to multi-atom interactions. The influence of detection efficiencies and collective effects on the appearance of trapping states at low temperatures is also found to be of particular importance.

Analytical solution of a one-dimensional lattice gas model with an infinite number of multiatom interactions.

We consider a one-dimensional lattice gas model in which the atoms interact via an infinite number of cluster interactions within contiguous atomic chains plus the next-nearest-neighbor pairwise interaction. All interactions are of arbitrary strength. An analytical expression for the size distribution of atomic chain lengths is obtained in the framework of the canonical ensemble formalism. Application of the exact solution to the problems of self-assembly and self-organization is briefly discussed.

A lattice gas model of strained epitaxy and self-organization of small clusters

We consider the Frenkel–Kontorova model of coherent strained epitaxy. We show that the model can be mapped onto a lattice gas model with cluster multiatom interactions. The latter is solved exactly in the case of pair, trio, and four-atom interactions. The method of solution can be extended to account for larger clusters. The solution shows the possibility of self-assembly of atoms into clusters with very narrow size distribution. The clusters themselves are distributed randomly at small covering but can be ordered at higher densities. In the two-dimensional (2D) case this may lead to surface reconstruction which is illustrated by Monte Carlo simulation of a 2D generalization of the model.

Influence of multiatom interactions on the shapes and energetics of two-dimensional homoepitaxial clusters on close-packed metallic surfaces.

We describe a scheme (BSM for ``bond saturation model'') for modeling the energetics of adatom clusters on close-packed metallic surfaces. In the BSM, atoms interact via coordination dependent nearest-neighbor bonds. We show that the BSM yields a detailed understanding of the relationship between shape and energy for two-dimensional (2D) homoepitaxial adatom clusters on close-packed metallic surfaces. In particular, we advance a simple rule for predicting the binding energies of small clusters based on the moments of their coordination distributions, derive a useful expression for the binding energy of a 2D cluster of arbitrary geometry, and study the role played by coordination dependent bonding in determining step energies for adatom islands. Our analysis of step energies provides insight into why the two distinct types of monatomic steps on Pt(111) have similar free energies as observed by Michely and Comsa. While our numerical results are for Pt(111), we expect our conclusions to hold for any chemically similar system. \textcopyright{} 1996 The American Physical Society.

On the stability of intermetallic phases

A general methodology using atomic clusters is applied to three problems connected to the study of alloy phase stability. The cluster method proposed by Allen and Cahn is applied to non-ideal hcp structures under tetrahedral approximation using multiatom interactions. The possible ground-state structures which are stable at absolute zero temperature are obtained. A geometrical representation in 4D parameter space of the possible strengths of multiatom interactions permitted for these structures is illustrated in terms of a 2D analogue. Extending these ideas, the cluster variation method (CVM) proposed by Kikuchi is applied to fcc structures under tetrahedral approximation to find the effect of multiatom interactions on the topology of the coherent phase diagrams in which all the phases present are derivable by mere rearrangement of atoms on the parent disordered structure. In addition, the possible invariant reactions are identified in such coherent phase diagrams. Finally the CVM is applied for calculating a model incoherent phase diagram, that of Ti-Zr system, where disordered hcp and bcc phases are present. The free energies of hcp and bcc phases are formulated using CVM procedures respectively under tetrahedral-octahedral and tetrahedral approximations. The CVM is shown to be in better agreement with the thermodynamic data and to be able to reproduce the correct value of measured enthalpy of transformation compared to that given by the regular solution model, which significantly overestimates the same.

Ground state structures in binary h.c.p. alloys using multiatom interactions

Abstract The cluster method is applied to binary (A-B) h.c.p. alloys having a non-ideal axial ratio in the tetrahedral approximation. Eight distinct cluster configurations are identified by populating A and B atoms on the distorted tetrahedral motif sites. The fractions of these clusters are the corresponding cluster variables. The configurational energy of the alloy is expressed as a function of four tetrahedral multiatom (four-body) interaction energy parameters, an effective pair interchange energy parameter and the cluster variables. The energy of the alloy is minimized for all possible triplets of clusters using the linear programming method. From the inequalities that minimize the energy and thus define the ground state, the permissible values of the multiatom interaction parameters in each case are clearly specified by a suitable geometric representation in a four-dimensional hyperspace spanned by the multiatom interaction parameters. Several ground state structures are obtained and the observed su...

Calculation of phase diagrams of binary fcc and ideal hcp alloys undergoing ordering transitions

The tetrahedron approximation of the cluster variation method (CVM) has been employed to investigate phase diagrams having fcc-based ordered and disordered phases. This approximation is also applicable to the binary hcp ordered structures with ideal axial ratio. The CVM developed by Kikuchi consists of calculating approximate expressions for the number of configurations and hence entropy of a crystal lattice having definite distribution of clusters (points, pairs, triangles, tetrahedra, etc.) of lattice points which in general may be occupied by one of a given set of atomic species. Tetrahedral multi-atom interactions denoted by α and β are utilized for expressing the configurational energy. The equilibrium cluster distribution is then found by minimizing the free energy by utilizing the natural iteration method developed by Kikuchi. The effect of α and β parameters on the topology of the resulting phase diagrams is observed by assigning several negative and positive values to them. The invariant reactions were also determined in each case. Finally a study was made on the Cd-Mg diagram.

Nonlinear dynamics of cooperative reconversion

We describe several new aspects of light–matter interactions for solids in which interatomic coupling of impurity atoms plays a dominant role in population dynamics. We explore the implications of spatial coherence in such multiatom interactions by introducing a density-matrix theory of cooperative upconversion, focusing on pair systems for which analytic results can be obtained. We predict population pulsations in coherent cooperative upconversion, enhanced quantum efficiency, enhanced energy transfer, and pair-mediated instabilities, not only in cooperative upconversion media without external cavities but in upconversion lasers and conventional lasers in highly doped solids as well. These predictions are compared with rate equation solutions and observations in lasers with inversions sustained by cooperative processes, particularly the 2.8-μm Er laser. Rate equations fail to predict the observed steady-state instabilities of this laser, which are well reproduced by density-matrix theory, furnishing evidence of weak coherent delocalizations in a rare-earth system.

Comparison of the stability of small clusters of hydrogen near a Pd(001) or a Ni(001) surface

Experimental data show that the H/Pd(001) system presents an ordered phase while a more diffuse diagram is observed for H/Ni(001). Using Green-function properties, we developed recently a formalism to describe the electronic structure of finite size hydrogen clusters near a metallic surface. Any atomic configuration can be considered in this real-space tight-binding framework. Multi-atom interactions are obtained as global quantities, getting round the explicit computation of differences between large numbers. Using this formalism, we present a comparison of the stability of small clusters of hydrogen near a Pd and a Ni surface. The stages of the growth in the case of H in Pd are described, and very good agreement with experimental results is obtained. In the case of H in Ni, the multi-atom interaction energies are very small, as observed experimentally.

Phase Transformations in Hexagonal-Close-Packed Alloys: Analysis with the Cluster Variation Method

ABSTRACTWe present a study of the hexagonal close-packed (hcp) Ising model for binary alloys within the cluster variation approximation. Groundstates of order stabilized by nearest-neighbor (NN) pair, triplet, and tetrahedron interactions were determined using the cluster configuration polyhedron method; no previous hcp ground-state study has considered all of these interactions. We predict physically realizable groundstates with stoichiometries A, AB (3 distinct structures), A2B, A3B, and A4B3. The previously unpredicted A4B3 structure is stabilized by multiatom (i.e. triplet, tetrahedron) interactions, while the others are stabilized by the two NN pair interactions. The Cluster Variation Method (CVM) was used to calculate the finite-temperature phase-equilibria for prototypical binary alloys. We present the first ordering phase diagrams computed with the CVM which contain all relevant groundstates for both isotropic and anisotropic NN pair interactions.

Phase stability and short range order in magnetic transition metal alloys: I. Electronic structure and interplay between chemical and magnetic cluster interactions

The role of the interplay between magnetism and chemical order on phase stability and chemical short range order, in transition metal alloys, has been investigated by means of the generalized pertubation method. This method, we introduced previously for non-magnetic systems, is now extended to take account of itinerant magnetism. The electronic energy of any alloy configuration, characterized by a given atomic distribution and a given distribution of magnetic moments, is then determined from a rapidly convergent cluster expansion. In a first step, it is shown that for ferromagnetic alloys magnetism induces, through a very simple mechanism, the stability of most of the atomically ordered structures observed in alloys with magnetic transition metals (Cr to Ni). In a second step, the structure of the cluster expansion for antiferromagnetic alloys in the ground state and more generally for magnetic alloys at finite temperatures is discussed in the framework of itinerant magnetism in the static approximation. In this first paper devoted to magnetism and order in transition metal alloys, we show that the generalized perturbation method enables to define chemical and magnetic pair and multiatom interactions from the electronic structure. As a result of the angular dependence of the transfer integrals it is shown that a simple extension of the theory for non-magnetic alloys allows to derive the most important chemical and magnetic interactions coming from “d” bands; as in non-magnetic systems, these interactions do not correspond to the most compact clusters (triangles, tetrahedron, …) but to straight self-retraced paths. The pair approximation and the relative importance of the multiatom chemical and magnetic interactions are discussed in view of the numerical results obtained in a simple tight binding scheme for “d” bands. For fcc and bcc lattices, the hierarchy of the cluster interactions allows to understand, at least qualitatively, the nature and the origin of the observed antiferromagnetic ordered structures and of the multiple spin density waves observed in some transition metal alloys.

Multiatom interactions between hydrogen atoms near a metallic surface

An electronic calculation of the interactions between hydrogen clusters near a metallic surface is presented. For the first time, tetramers of impurities are considered. The interaction energies are determined in a self-consistent way within the tight-binding formalism. This approach is applied to clusters of hydrogen near a Pd(001) surface. Agreement with experimental results is good: for an adsorbed layer, multiatom interactions stabilize the observed C(2 × 2) structure. In the bulk the HH distance is greater than 2.1 Å. The determination of the multiatom interactions is needed for the overlayer phase diagram calculation. Such cluster interactions cannot be reduced strictly to a combination of pair energies and lead to an asymmetrical phase diagram.

Hydrogen trimers and tetramers near a Pd(001) surface

We compute directly the multi-atom interactions of small clusters near a metallic surface within the tight binding scheme. We study the stability of the system of these interacting impurities as a function of their relative position in the semi-infinite host. This model is applied to a trimer and tetramer of hydrogen near a Pd(001) surface and we find good agreement with the experimental results. This work indicates that the multi-atom interactions, in the dilute limit, can be, only in a very approximate way, written as the sum of the relevant pair interactions and that cluster interactions may be dependent on the hydrogen concentration.

Multiatom interactions between hydrogen atoms near a metallic surface

An electronic calculation of the interactions between hydrogen clusters near a metallic surface is presented. For the first time, tetramers of impurities are considered. The interaction energies are determined in a self-consistent way within the tight-binding formalism. This approach is applied to clusters of hydrogen near a Pd(001) surface. Agreement with experimental results is good: for an adsorbed layer, multiatom interactions stabilize the observed C(2 × 2) structure. In the bulk the HH distance is greater than 2.1 Å. The determination of the multiatom interactions is needed for the overlayer phase diagram calculation. Such cluster interactions cannot be reduced strictly to a combination of pair energies and lead to an asymmetrical phase diagram.