Molecular dynamics study of the thermal behavior of nanometer-sized au hollow cubes

Molecular dynamics simulations have been employed to study the thermal response of an Au hollow cube with a side about 8 nm long and walls about 3 nm thick. The gradual temperature rise determines the occurrence of a hierarchical sequence of melting transitions regarding atoms with a progressively increasing number of nearest neighbors. Atoms located at the cube edges are thus seen to undergo melting first, then followed by surface and grain boundary species, and finally by atoms in bulklike regions of cube walls. A percolating liquidlike framework connecting external and internal surfaces is formed in the temperature range within which grain boundaries are partially molten. Such framework represents a preferential diffusion path for liquidlike species.     

Generation and characterization of low‐energy structures in atomic clusters

Factors relevant for controlling the structures determined in the local optimization of argon clusters are investigated. In particular, the role of volume and shape for the box where initial structures are generated is assessed. A thorough characterization of the optimization is also presented, based on a nearest‐neighbor analysis, in clusters ranging from 30 to 55 atoms. This includes the assessment of the degree of preservation of aspects of the initial randomly generated structure in the final optimized counterpart, and the correlation between optimized energy and the number of nearest neighbors and average departure from the diatomic reference distance. The usefulness of this analysis to explore the energy landscape of atomic clusters is also highlighted. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

FLAPW方法研究δ-钚单层表面几何和电子结构

采用全势线性缀加平面波（FLAPW）方法, 在广义梯度近似（GGA）下研究了单层δ-钚（100）和（111）面的几何和电子结构. 结果表明,自旋-轨道耦合对平衡几何结构的影响强于自旋极化,与体相相比,表面晶格常数显著收缩,实验上可选择晶格常数相对较小的物质作为δ-钚薄膜的沉积基体. 表面原子由于近邻原子数目的减少,5f电子波函数重叠几率的降低,更多地表现出局域化特征,（100）面5f电子的局域化程度强于（111）面.     

Ab initio SCF calculations of the linear infinite chain of LiH according to the pseudo-lattice method

The pseudo-lattice (PL) method has been reformulated for ab initio self-consistent-field (SCF) calculations. The translational symmetries of infinite systems have been applied to the finite model chain by manipulating all the intramolecular and intermolecular Fock matrices. The nuclear repulsion energy has been corrected accordingly. The method has been tested for the linear chain of lithium hydride under the constraint of equidistance between all neighboring lithium and hydrogen atoms. The calculated results of the infinite chain have been compared with those of finite chains of lithium hydride under the same geometric constraint. The equilibrium geometries, band structures, intermolecular stabilization energies and potential curves have been studied. It is found that the infinite systems cannot be described by considering only first nearest neighbor interactions, and the intermolecular interactions must be considered at least up to third nearest neighbors in order to obtain accurate value of force constant of infinite systems. We can conclude from band structures of infinite chains that the boundary effect of the finite model chain is effectively removed by the PL method.     

Ground‐state structures of polymers

Two‐ and three‐dimensional structures A_xB_y can be characterized by the numbers T₁ and T₂ of nearest and next‐nearest neighbors of the same kind. A small number of structures at the border of the T₁, T₂ structure map is stabilized by enthalpy compared to an increased number of entropy stabilized structures. About 60 three‐dimensional structures with T₁=2 nearest neighbors of all A atoms are suitable for infinite chains of polymers like (CH)_∞, (CHCH₂)_∞, or (CH₂)_∞. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1944–1955, 2001     

Formation of iron fine particles in a crystal lattice by the computer simulation

The growing up of the fine particles in a sapphire crystal was simulated with a computer and the expected Mössbauer spectra were calculated by considering the influence of the nearest neighbors and next-nearest neighbors to the magnetic hyperfine field of the iron atoms. The simulated spectra were compared with the Mössbauer spectra observed with a Fe-implanted sapphire and the agreement between them was satisfactory on the whole. It is also revealed that an iron atom implanted into a sapphire crystal may interact, before coming to rest, with the iron fine particles situated within 2–3 nm in the crystal.     

Analysis of the linear tension of two-dimensional droplets on heterogeneous adsorbents

A procedure for analyzing the formation processes of two-dimensional droplets of an adsorbate on a rigid adsorbent support is considered. The molecular theory is based on data on the potential functions between adsorbent atoms and adsorbate molecules. Interactions between nearest neighbors are considered in the quasi-chemical approximation. The internal motions of adsorbent atoms and adsorbate molecules are ignored. Problems of describing the formation of droplets on heterogeneous adsorbents are associated with calculations for binodals (illustrated with the simplest example of two different homogeneous crystal faces) due to the choice of methods for calculating linear tension and the structural model of the region of the liquid–vapor transition. The dependence of the characteristics of droplets in the layered structural model on the method for determining the reference lines of the tension is shown for their metastable and equilibrium states. It is found that for a number of structural parameters, the thermodynamic determination of the line of tensions of metastable droplets can result in nonmonotonic dependences of the linear tension on their radii. The characteristics of two-dimensional liquid–vapor interfaces are compared for two structural models: coordination sphere and layered. It is found that the coordination sphere model allows the exclusion of the structural parameter of the layered model, but both models need refinement at small radii.     

Computer simulation of the structure of water clusters containing absorbed ethane molecules

The structure of water clusters that have absorbed ethane molecules is studied by the molecular dynamics method. Structural analysis is performed by the construction of Voronoi polyheda for oxygen atoms and hybrid polyheda whose centers coincide with the centers of oxygen atoms and the faces are formed according to the positions of hydrogen atoms. The (H₂O)₂₀ cluster can retain no more than four ethane molecules remaining at the same time stable. When a water cluster adds more than four ethane molecules, the volumes of Voronoi polyheda acquire values close to the volume per molecule in the bulk liquid water. As the number of ethane molecules in a water cluster increases, the number of hydrogen atoms adjacent to oxygen, as well as the average number of units in cyclic formations composed of hydrogen atoms, also increases. In this case, the number of H-O-H angles formed by the nearest geometric neighbors close to 89° becomes dominant. The coefficient of nonsphericity reflecting the local arrangement of hydrogen atoms around the oxygen atoms decreases as the C₂H₆ molecules are added to water cluster and approaches to the value of this coefficient for the rhombic dodecahedron in the case of adsorption of six ethane molecules.     

Origin of the quadrupole splitting in the Mössbauer 57Fe spectrum of cubic (disordered) LiFeO2

The room-temperature Mössbauer ⁵⁷Fe spectrum of polycrystalline disordered cubic α-LiFeO₂ contains a quadrupole splitting |Δ_obs| of 0.65(2) mm/sec. This value is relatively large for an Fe atom in an essentially Fe³⁺_HS state. To account for its magnitude, the distribution of the electric-field gradient (EFG) values associated with the Fe atoms was investigated by means of exact geometric analysis involving the 12 nearest cation neighbors (model A) as well as large-scale computer simulation involving more distant cations (models B to E). It is found that (1) the major contribution to |Δ_obs| comes from the distribution of +1 and +3 charges among the 12 nearest cation neighbors of a reference Fe atom; (2) this contribution by itself largely accounts for |Δ_obs|; (3) the contribution from cations beyond the seventh-nearest neighbors is marginal; (4) displacing the oxygen atoms from their lattice sites toward adjacent Fe atoms produces a significant effect on the distribution of EFG values at a reference Fe atom, while incipient cation ordering appears to have a relatively small effect; and (5) the contribution of the EFG = 0 component to model |EFG| distributions will be overemphasized unless cations beyond the first-nearest neighbors are included in the EFG summation. The 144 distinct (up to rotation and reflection) Li¹⁺_12?kFe³⁺_k configurations on the coordination cuboctahedron of nearest cation neighbors (required for the examination of model A) are listed, together with their symmetries and multiplicities, and it is shown that the 144 configurations engender only 17 distinct |EFG| values. Observations are also made on various geometric aspects of calculating EFG at ⁵⁷Fe³⁺_HS on cubic lattices.     

A theory of a curved vapor-liquid separation boundary: The lattice gas model

Molecular theory of curved vapor-liquid interphase boundaries was considered in terms of the lattice gas model. The theory uses the quasi-thermodynamic concept of curved layers of a separation boundary with a large radius. The transition from a rectangular lattice to such layers is performed by the introduction of a variable number of the nearest neighbors. The problems (1) of the transition from distributed molecular models to layer models reflecting macroscopic symmetry of the interphase boundary and (2) of a minimum linear size of the surface region to which thermodynamic approaches are applicable were considered. Equations for the quasi-equilibrium distribution of molecules at the vapor-liquid boundary in a metastable system were constructed in the quasi-chemical approximation taking into account direct correlations between the nearest interacting molecules. A metastable state is maintained by a pressure jump described by the macro-scopic Laplace equation on a separation surface inside the interphase region. Equations for local mean pressure values and normal and tangential pressure tensor components inside the interphase region were constructed. These equations were used to obtain microscopic difference mechanical equilibrium equations for curved boundaries of spherical and cylindrical drops in the metastable state. The relation between the micro-scopic difference mechanical equilibrium equations and similar differential equations and the macroscopic Laplace equation, which described pressure jump in a metastable system, was considered. Various definitions of surface tension are discussed.     

Reactions of adjacent substituents on polymer chains: Combinatorial consequences of the mechanism

This paper deals with a statistical problem arising from the pairwise reaction of immediately adjacent substituents along the backbone of a linear polymer chain. The possibility arises that a given substituent becomes unavailable for reaction as a result of the reaction of its nearest neighbors on either side. Previous treatments of this problem have not explicitly taken the consequences of the reaction mechanism into account. In this paper, it is shown that the expected number of unreacted substituents remaining after exhaustive pairwise reaction is a function of the reaction mechanism. For example, in the case of the chain model adopted here, which corresponds to a perfectly regular head-to-tail vinyl halide (-CH₂-CHX-), we show that the fraction of halide atoms remaining after exhaustive removal is equal to 0.1233. The result is compared with results obtained from previous work.     

Equation of state for hard-sphere fluid in restricted geometry

Many practical applications require the knowledge of the equation of state of fluids in restricted geometry. We study a hard-sphere fluid at equilibrium in a narrow cylindrical pore with hard walls for pore radii R<((square root 3)+2)/4 (in units of the hard sphere diameter). In this case each particle can interact only with its nearest neighbors, which makes possible the use of analytical methods to study the thermodynamics of the system. Using a transfer operator formalism and expanding in low- and high-pressure regions, we can obtain a simple analytical equation of state for almost all ranges of pressure. The results agree with Monte Carlo simulations. Additionally, it is shown that a convenient analytical representation can be chosen to accurately describe the equation of state within the error of the Monte Carlo simulation.     

Monte Carlo simulations for 1- and 2D spin crossover compounds using the atom–phonon coupling model

In this contribution we have simulated, using the Monte Carlo–Metropolis technique, the thermal behavior of an one dimensional and of a square lattice spin crossover system.In the one dimensional case, a long-range interaction parameter must be included in order to obtain a thermal hysteresis. For the square lattice we have tacked into account the nearest as well the next-nearest neighbors. We show the role of the elastic constant ratio on the hysteresis width.     

A complete nearest-neighbor force field model for C60

A force field model is developed for C(60) that features 13 force constants representing all interactions between nearest-neighboring atoms. The model is compared with, and tested against, other force field models in the literature. Force constants for C(60) are then deduced by fitting the model to the 14 known optically accessible vibrational frequencies of the molecule. Finally, the model is fitted to two existing theoretical calculations of the complete vibrational spectrum of C(60). Fair agreement is obtained with the theoretical calculations, implying that interactions with atoms other than nearest neighbors are small.     

A parameter-free, solid-angle based, nearest-neighbor algorithm

We propose a parameter-free algorithm for the identification of nearest neighbors. The algorithm is very easy to use and has a number of advantages over existing algorithms to identify nearest-neighbors. This solid-angle based nearest-neighbor algorithm (SANN) attributes to each possible neighbor a solid angle and determines the cutoff radius by the requirement that the sum of the solid angles is 4π. The algorithm can be used to analyze 3D images, both from experiments as well as theory, and as the algorithm has a low computational cost, it can also be used "on the fly" in simulations. In this paper, we describe the SANN algorithm, discuss its properties, and compare it to both a fixed-distance cutoff algorithm and to a Voronoi construction by analyzing its behavior in bulk phases of systems of carbon atoms, Lennard-Jones particles and hard spheres as well as in Lennard-Jones systems with liquid-crystal and liquid-vapor interfaces.     

Analysis of the electron-beam radiation damage of TEM samples in the acceleration energy range from 0.1 to 2 MeV using the standard theory for fast electrons

The electron-beam–sample interaction is analyzed using the standard theory for fast electrons in the accelerating energy range from 0.1 to 2 MeV when the sample to be observed with TEM is composed of different atoms. This theory allows taking into account the contribution of the nearest neighbors of the target atoms, which is a more real approximation. For direct interaction the normal expressions are obtained, but for the cascade phenomenon a better approximation is presented. This theory is applied to the analysis of the experimentally reported electron-beam-induced structure modification in the superconductor YBa₂Cu₃O_7−x, the quasicrystalline alloy Al₆₂Cu₂₀Co₁₅Si₃, and the tooth enamel hydroxyapatite.     

EXAFS studies of nickel nafion ionomers

Nafion membranes neutralized with Ni²⁺ have been examined by extended x-ray absorption fine-structure (EXAFS) and x-ray absorption near-edge-structure (XANES) spectroscopy. The results indicate that in both the dry and water-soaked membranes, the nickel is in an octahedral site with six oxygen atoms as nearest neighbors. The degree of disorder in the Ni? O distance is comparable to that in ionic crystals in both the dry and hydrated materials. A contribution from a second shell of neighbors is very weak in the dry samples but, surprisingly, this contribution is strongly accentuated in the hydrated membranes. The data indicate that this contribution is due to neighboring Ni²⁺ cations. Thus the water absorption seems to enhance the local ordering of the cation environment. The local structure does not depend strongly on the concentration of ionic groups in the materials.     

Local structural analysis of In-doped Bi2Se3 topological insulator using X-ray fluorescence holography

We performed X-ray fluorescence holography measurements on an In-doped Bi₂Se₃ topological insulator and obtained an in-plane atomic image in the vicinity of In. We found that atomic images at the positions of the first nearest neighbors (NNs) are very weak whereas those at the positions of the second and the third NNs are relatively strong. On the basis of the fact that In is half of the atomic number of Bi, we attributed the origin of this feature to the clustering of the In atoms in the Bi plane. We calculated the intensity of the atomic images and confirmed that the formation of In cluster results in a decrease by 30% in the first NN atomic image intensity. However, the decrease in the magnitude is not enough to explain the experimental results, suggesting another contribution such as the lattice distortions. The effect of the lattice distortion on the atomic image intensity is discussed on the basis of the simulation including the positional fluctuation of In atoms.     

Probability of fluctuations of the number of nearest neighbors in a hard sphere liquid: Asymptotic decomposition

Fluctuations of the number of nearest neighbors M _λ inside the coordination sphere of a given radius λ, in a hard sphere liquid (M _λ is the total number of neighbors for all N particles of the system) are analyzed. For the probability density distribution of this random value, an exact asymptotic decomposition over the number of particles N in the system has been derived, and the decomposition coefficients were recorded in terms of the semi-invariants of the random value of Mλ The result coincides completely with the asymptotic decomposition in the central limiting theorem of the probability theory. Due to this, the number of nearest neighbors in a liquid of N hard spheres can be represented as the sum of N independent random values with equal distributions. The hard sphere liquid of N particles was simulated by the Monte Carlo method in the canonical Gibbs ensemble over a wide range of densities (N= 864, periodic boundary conditions, occupancy pg from 0.005 to 0.500). The theoretical equations are in good agreement with the results of simulation.     

A universal description of a solid crystal and liquid using discrete distribution functions

The numerical implementation of a crystallization/melting theory that has two branches of isotherms for liquids and solids as in Landau’s theory within the general self-consistent theory of condensed states is discussed. Equations are based on discrete distribution functions (the lattice gas model). Molecular distributions are calculated to a quasi-chemical approximation reflecting the effects of direct correlations between interacting particles. the interaction between particles is described by the Lennard-Jones potential, while the vibrational motion of molecules is included using a modified quasi-dimer Mie model. The effect of the vibrations of atoms on lateral interactions between nearest and next nearest neighbors is considered, along with a scheme for considering the displacement of particles in a liquid that modify such lateral interactions. Examples of calculations describing a defective crystal and vapor-liquid system are given.