A Lennard-Jones Embedded-Atom Potential and Its Application to the Study of Melting

The nearest-neighbour Lennard-Jones potential from the embedded-atom method is extended to a form that includes more than nearest neighbours. The model has been applied to study melting with molecular dynamics. The calculated melting point, fractional volume change on melting, heat of fusion and linear coefficients of thermal expansion are in good agreement with experimental data. We have found that the second and third neighbours influence the melting point distinctly.

An extension of the Quasicontinuum Treatment of Multiscale Solid Systems to Nonzero Temperature

Covering the solid lattice with a finite-element mesh produces a coarse-grained system of mesh nodes as pseudoatoms interacting through an effective potential energy that depends implicitly on the thermodynamic state. Use of the pseudoatomic Hamiltonian in a Monte Carlo simulation of the two-dimensional Lennard-Jones crystal yields equilibrium thermomechanical properties (e.g., isotropic stress) in excellent agreement with ``exact'' fully atomistic results.

A Generalized Soft-Sphere for Monte Carlo Simulation

A new collision model, called the generalized soft-sphere (GSS) model, is introduced. It has the same total cross section as the generalized hard-sphere model [Phys. Fluids A 5, 738 (1993)], whereas the deflection angle is calculated by the soft-sphere scattering model [Phys. Fluids A 3, 2459 (1991)]. In virtue of a two-term formula given to fit the numerical solutions of the collision integrals for the Lennard-Jones (6-12) potential and for the Stockmayer potential, the parameters involved in the GSS model are determined explicitly that may fully reproduce the transport coefficients under these potentials. Coefficients of viscosity, self-diffusion and diffusion for both polar and nonpolar molecules given by the GSS model and experiment are in excellent agreement over a wide range of temperature from low to high.

Molecular Dynamics Simulation of Barnacle Cement

Barnacle cement is an underwater adhesive that is used for permanent settlement. Its main components are insoluble protein complexes that have not been fully studied. In present article, we chose two proteins of barnacle cement for study, 36-KD protein and Mrcp-100K protein. In order to investigate the characteristic of above two proteins, we introduced the method of molecular modeling. And the simulation package GROMACS was used to simulate the behavior of these proteins. In this article, before the simulations, we introduce some theories to predict the time scale for polymer relaxation. During the simulation, we mainly focus on two properties of these two proteins: structural stability and adhesive force to substrate. First, we simulate the structural stability of two proteins in water, and then the stability of 36-KD protein in seawater environment is investigated. We find that the stability varies in the different environments. Next, to study adhesive ability of two proteins, we simulate the process of peeling the two proteins from the substrate (graphite). Then, we analyze the main reasons of these results. We find that hydrogen bonds in proteins play an important role in the protein stability. In the process of the peeling, we use Lennard-Jones 12-6 potential to calculate the van der Waals interactions between proteins and substrate.

Mechanisms underlying two kinds of surface effects on elastic constants

ABSTRACT Recently, people are confused with two opposite variations of elastic modulus with decreasing size of nano scale sample: elastic modulus either decreases or increases with decreas- ing sample size. In this paper, based on intermolecular potentials and a one dimensional model, we provide a unified understanding of the two opposite size effects. Firstly, we analyzed the mi- crostructural variation near the surface of an fcc nanofilm based on the Lennard-Jones potential. It is found that the atomic lattice near the surface becomes looser in comparison with the bulk, indicating that atoms in the bulk are located at the balance of repulsive forces, resulting in the decrease of the elastic moduli with the decreasing thickness of the film accordingly. In addition, the decrease in moduli should be attributed to both the looser surface layer and smaller coor- dination number of surface atoms. Furthermore, it is found that both looser and tighter lattice near the surface can appear for a general pair potential and the governing mechanism should be attributed to the surplus of the nearest force to all other long range interactions in the pair po- tential. Surprisingly, the surplus can be simply expressed by a sum of the long range interactions and the sum being positive or negative determines the looser or tighter lattice near surface re- spectively. To justify this concept, we examined ZnO in terms of Buckingham potential with long range Coulomb interactions. It is found that compared to its bulk lattice, the ZnO lattice near the surface becomes tighter, indicating the atoms in the bulk located at the balance of attractive forces, owing to the long range Coulomb interaction. Correspondingly, the elastic modulus of one- dimensional ZnO chain increases with decreasing size. Finally, a kind of many-body potential for Cu was examined. In this case, the surface layer becomes tighter than the bulk and the modulus increases with deceasing size, owing to the long range repulsive pair interaction, as well as the cohesive many-body interaction caused by the electron redistribution.

伦纳德琼斯薄膜材料导热系数的分子动力学研究

提出从微观的角度,借助计算机工具,将薄膜破坏发展的细节展现出来的分子动力学研究的思想。使得实验上难以观察的现象变得形象而便于理解。应用分子动力学理论,使用伦纳德琼斯势函数,采用预校正积分法和虚拟外力约束标定方法,模拟薄膜体系的传热系数受体系的密度、温度的影响,同时结合体系粒子的径向分布函数和长程分布函数分析了相应的系统结构特性。另外,采用不同的模拟尺寸获得了低维材料所特有的“高温尺寸效应”。结果显示,导热系数随密度的增加变大,随温度的上升而变大。这些数据现有测量手段是难以得到的,这类模拟可以为研究提供一些

Phase equilibria and plate-fluid interfacial tensions for associating hard sphere fluids confined in slit pores

The excess Helmholtz free energy functional for associating hard sphere fluid is formulated by using a modified fundamental measure theory [Y. X. Yu and J. Z. Wu, J. Chem. Phys. 117, 10156 (2002)]. Within the framework of density functional theory, the thermodynamic properties including phase equilibria for both molecules and monomers, equilibrium plate-fluid interfacial tensions and isotherms of excess adsorption, average molecule density, average monomer density, and plate-fluid interfacial tension for four-site associating hard sphere fluids confined in slit pores are investigated. The phase equilibria inside the hard slit pores and attractive slit pores are determined according to the requirement that temperature, chemical potential, and grand potential in coexistence phases should be equal and the plate-fluid interfacial tensions at equilibrium states are predicted consequently. The influences of association energy, fluid-solid interaction, and pore width on phase equilibria and equilibrium plate-fluid interfacial tensions are discussed.

高温高压气体状态方程研究及钱学森方程改进

钱学森根据分子动理论的普遍原则,结合Lennard-Jones Devanshire(LJD)液体理论的结果,给出了一个适用于高温高压气体的普适状态方程.该方程有物理基础,且具有计算精度高、形式简单及显含温度的优点.本文用分子动力学方法检验了LJD理论的适用性,并给出精确的高温高压气体状态方程分子动力学数值解,在此基础上修订了钱学森状态方程.改进后的钱学森状态方程在高密度范围内与MD结果平均绝对偏差率小于10%.

原子力显微镜Tapping模式探针动力学与操作方法研究

自从1986年G. Binnig 等发明原子力显微镜(AFM)以来，纳米科技研究得到了快速发展，纳米科技研究的最终目标是从纳米甚至分子与原子尺度上制造功能器件或系统，而实现此目标的手段之一是研制能够在纳米尺度上进行精确可控操作的装置、方法与技术。对此，学者们对基于 AFM 的纳米操作进行了大量研究。基于AFM-Tapping模式的纳米观测与操作具有成本低、易实现性等特点，因而近年来成为纳米技术研究的热点。但基于AFM探针的纳米观测和操作的基本理论仍然很不完善，许多情况下缺乏基于探针观测与操作的纳观作用力的合理解释与描述，因此限制了基于探针的纳米技术发展。为了解决这一问题，本论文在国家自然科学基金项目“基于机器人化的纳米操作方法研究(60575060)”的支持下，广泛收集和查阅国内外相关资料，重点开展了AFM-Tapping模式下探针动力学解释方法与建模研究，并进行了基于AFM-Tapping的纳米操作方法研究。主要研究工作如下： 1)针对探针动力学建模问题，开展了基于Hamilton原理和变分原理的有边界条件的探针一维悬臂梁建模方法研究。利用模态叠加法和模态函数正交性对探针各阶模态进行了解耦分析，得到了解耦的探针动力学方程。 2)开展了纳观尺度下探针-样品作用力研究。分析与阐述了毛细力、接触力和摩擦力等纳观尺度作用力；开展了基于Lennard-Jones势模型的不同形状探针与样品间的相互作用势描述方法研究；并根据势能-力之间的关系，研究了可描述半球形探针与样品间相互作用力的数学表达式。 3)开展了基于弹簧振子模型的纳观非线性力作用下探针振动问题分析研究。首次提出了基于探针弹性力与非线性纳观力平衡关系的探针振动状态区域划分方法。在探针振动能量平衡分析的基础上，研究了AFM-Tapping模式中的双稳态现象，进而得到了描述双稳态现象的解析表达式。最后采用Lyapunov指数分析的方法，证明了探针在非线性振动中确实存在着混沌现象。 4)研究了基于AFM-Tapping模式的纳米操作方法。通过研究探针振幅调节控制和预编程作业轨迹规划策略，提出了一种具有增强探针概念的纳米作业方法。该方法可以在参数设定和预编程条件下自动完成纳米尺度的平移和旋转操作，从而可以大大提高纳米操作的效率和精度。实验结果验证了该方法的有效性。本文的工作为纳米操作方法的研究提供了可以借鉴的理论与实验经验，有助于推动基于AFM的纳米操作与制造技术的发展。

Characterization Of Crystal Structure In Binary Mixtures Of Latex Globules

Colloidal crystals formed by two types of polystyrene particles of different sizes (94 and 141 nm) at various number ratios (94:141 nm) are studied. Experiments showed that the formation time of crystals lengthens as the number ratio of the two components approaches 1:1. The dependence of the mean interparticle distance (D-0), crystal structure and alloy structure on the number ratio of the two types of particles was Studied by means of Kossel diffraction technique and reflection spectra. The results showed that as the number ratio decreased, the mean interparticle distance (D-0) became larger. And the colloidal crystal in binary mixtures is more preferably to form the bcc structure. This study found that binary systems form the substitutional solid solution (sss)-type alloy structure in all cases except when the number ratio of two types of particles is 5:1, which results instead in the superlattice structure. (C) 2008 Elsevier Inc. All rights reserved.