网格上布鲁塞尔体系化学振荡的粗粒化模拟

将一种有效的粗粒化的动力学蒙特卡罗(KMC)方法用于加速模拟二维格气布鲁塞尔体系中的振荡行为．这种方法是将微观网格合并得到粗粒化的网格,并在该粗粒化网格上按粗粒化的反应速率执行KMC,即粗粒化的KMC．数值结果表明,由于非线性三分子反应导致的相邻元胞之间的关联是不能忽略的．通过正确的考虑这一边界效应,引入了所谓的b-LMF方法．大量的数据表明,只要体系的扩散系数不是很小,b-LMF方法能够很好的重现体系的振荡行为．另外,发现该方法所得的结果与KMC的偏离在合适的粗粒化尺度下存在一个接近于0的极小值,这一粗     

Fe-Cr二元合金微观组织演化的质量密度场耦合动力学Monte-Carlo模拟研究

本文建立了一种全新的将动力学Monte-Carlo粒子模拟与基于归一化Gauss函数基组的质量密度场空间粗粒化模型耦合的杂化模拟算法.采用该杂化模拟算法,系统对比研究了4种Cr原子含量分别为12.8%,20.0%, 30.0%和40.0%的Fe-Cr合金中Cr相在温度为673 K下的时效析出动力学机制,及其时效不同阶段微观组织形貌的演变规律.研究得出Fe-Cr (12.8%)合金富Cr相时效组织形貌呈现孤立颗粒状空间分布形态,时效机制属于形核-长大(NG)机制;对于Fe-Cr (30.0%)和Fe-Cr (40.0%),富Cr相时效形貌在形核-生长及熟化阶段均呈现为三维蠕虫状空间分布特征,时效机制属于条幅分解(SD)机制;对于Fe-Cr (20.0%)合金,其富Cr相组织演化特征介于NG和SD机制之间.研究进一步发现Cr原子短程序参量可用来分析富Cr相形核-生长阶段Fe-Cr合金原子尺度结构的演变,但对于时效熟化阶段微观结构组织变化不敏感.基于空间粗粒化后Fe-Cr合金微观组织形貌,进一步分析了4种Cr原子含量下Fe-Cr合金相变动力学参数如富Cr相体积分数、平均粒径及相颗粒数密度随时...     

基于Gay-Berne势能模型的粗粒化动力学模拟研究正丁醇的玻璃态相变

通过基于Gay-Berne势能模型的粗粒化动力学模拟,研究液态正丁醇体系的冷却过程. 采用密度泛函计算,拟合出适合正丁醇体系的GB势能参数. 体系的密度、平均势能等性质随温度的降低(由290 K降至50 K,间隔为10 K)发生特殊变化,即体系发生玻璃态相变,相变温度为T_g=120±10 K,与实验值110±1 K符合很好.     

与相变潜热有关的铁电-顺电相界动力学及其尺寸效应

对于铁电-顺电相界的快移动，提出了相变潜热释放传导机理进行解释，并对其尺寸效应做了进一步研究.研究表明相界移动速率随体系厚度的减小而增加.理论结果与实验符合. 关键词： 铁电相变 相界动力学 相变潜热 尺寸效应     

双喷注事件中单个喷注内部动力学起伏的蒙特卡洛研究

用蒙特卡洛方法研究了91.2GeV e⁺e^－碰撞产生的双喷注事件中单个喷注内部的动力学起伏.结果表明,喷注内部的动力学起伏的各向异性随着截断参数y_cut的变化而明显地改变.存在一个转变点,在该点处动力学起伏在纵横平面是各向异性的,而在横平面是各向同性的.由此得到了产生喷注的一个标度.     

相变材料热物理性质的分子动力学模拟

为从微观尺度探寻相变材料的热物性变化机理, 本文采用分子动力学的方法, 构建了由正二十二烷组成的无定形结构的相变材料体系, 采用周期性边界条件以及COMPASS力场对相变材料的比热以及导热系数进行了模拟, 并对纯正二十二烷进行了DSC测试. 结果表明, 模拟所得的相变材料热容与文献实验值的偏差是6.5%, 熔点与DSC实验值的偏差是0.98%. 当温度为288–318 K时, 相变材料的导热系数在0.1–0.4 W·m^-1·K^-1 范围内波动, 且随着压力增大略呈下降趋势. 关键词： 扩散系数 比热 导热系数 分子动力学     

生物大分子多尺度理论和计算方法

分子模拟是研究生物大分子的重要手段. 过去二十年来, 人们将分子模拟与实验研究相结合, 揭示出生物大分子结构和动力学方面的诸多重要性质. 传统分子模拟主要采用全原子分子模型或各种粗粒化的分子模型. 在实际应用中, 传统分子模拟方法通常存在精度或效率瓶颈, 一定程度上限制了其应用范围. 近年来, 多尺度分子模型越来越受到人们的关注. 多尺度分子模型基于统计力学原理, 将全原子模型和粗粒化模型相耦合, 有望克服传统分子模拟方法中的精度/效率瓶颈, 进而拓展分子模拟在生物大分子研究中的应用范围. 根据模型之间的耦合方式, 近年来发展起来的多尺度分子模拟方法可归纳为如下四种类型: 混合分辨多尺度模型、并行耦合多尺度模型、单向耦合多尺度模型、以及自学习多尺度模型. 本文将对上述四类多尺度模型做简要介绍, 并讨论其主要优缺点、应用范围以及进一步发展方向.     

单空位体系多尺度模拟

基于多尺度协同算法，实行了统一动力学模拟方案，研究了单空位体系的位移分布与位移振荡行为、能量随时间起伏以及差分电荷密度与外部环境的影响.计算结果发现位移分布及其相关行为反映了单空位体系的量子力学效应；同时，体系总能、差分电荷密度计算则给出了外部环境的影响，反映了经典效应与量子效应的耦合. 关键词： 多尺度杂化算法 统一动力学 电子态 原子位移     

感染生长模型的逾渗模拟

在规则格子点阵中，活跃点逐步动态地以可变概率感染附近空缺点而生成系综.利用感染概率替代系综温度，给粒子划分能级，可以用巨正则系综配分函数表征体系.蒙特卡洛方法模拟验证了该体系在逾渗阈值处的相变行为.提出了一种新的较为普适的估算规则点阵逾渗阈值的方法.对介质基底上金属薄膜的实验研究验证了该感染生长模型的合理性.由此给出了格子点阵的固有属性(逾渗)如何在粒子聚集成团簇这一动态过程中体现出来的物理模型. 关键词： 逾渗 系综 蒙特卡洛方法 生长模型     

微观尺度下金属/气体界面RM不稳定性 自相似现象的分子动力学模拟

极端冲击加载条件下的RM (Richtmyer-Meshkov)不稳定性在惯性约束核聚变领域有重要的学术价值和工程意义。宏观动力学方法受限于极端条件下的模型和参数准确性而难以直接应用,微观分子动力学方法则受限于计算量而难以直接模拟宏观尺度现象。为了解RM不稳定性微观与宏观规律之间的联系,采用基于嵌入原子多体势(EAM)的分子动力学方法模拟铜-氦微观尺度界面在极端冲击加载条件下(活塞冲击加载速度6～15 km/s)的RM不稳定性现象,对比文献提供的近似条件下宏观模拟结果发现,演化过程在唯象上完全相似。进一步比较了不同尺度(7.3～145.0 nm)、不同冲击加载速度(11.7～20.6 km/s)、不同初始界面扰动(扰动振幅与波长之比0.20～0.05)条件下振幅发展规律,发现在相同冲击动力条件和边界条件下,RM不稳定性的振幅增长规律在计算尺度范围内完全自相似,主要参数的变化特征符合理论预测。尽管理论模型因简化而存在一定偏差,但是微观模拟获得的振幅增长规律与宏观现象有相似的变化特征。     

Totally asymmetric exclusion processes at constrained m-input n-output junction points

Totally asymmetric exclusion processes at constrained m-input n-output junction points under random update are studied by theoretical calculation and computer simulation in this paper. At the junction points, the hopping rate of particles from m-input parallel lattices to n-output parallel lattices is assumed to be equal to r/n （0 〈 r 〈 1 ）. The mean-field approach and Monte Carlo simulations show that the phase diagram can be classified into three regions at any value of r. More interestingly, there is a threshold rc = n（ 1 - √1 - m/n）/m. In the cases of r 〉 re and r 〈 rc, qualitatively different phases exist in the system. With the increase of the value of m/n, the regions of （LD, LD） and （MC, LD） or （HD, LD） decrease, and the （HD, HD） is the only phase that increases in the region （LD stands for low density, HD stands for high density, and MC for maximal current）. Stationary current and density profiles are calculated, showing that they are in good agreement with Monte Carlo simulations.     

Multilevel coarse graining and nano-pattern discovery in many particle stochastic systems

In this work we propose a hierarchy of Markov chain Monte Carlo methods for sampling equilibrium properties of stochastic lattice systems with competing short and long range interactions. Each Monte Carlo step is composed by two or more sub-steps efficiently coupling coarse and finer state spaces. The method can be designed to sample the exact or controlled-error approximations of the target distribution, providing information on levels of different resolutions, as well as at the microscopic level. In both strategies the method achieves significant reduction of the computational cost compared to conventional Markov chain Monte Carlo methods. Applications in phase transition and pattern formation problems confirm the efficiency of the proposed methods.     

Short-time dynamics and critical behavior of three-dimensional bond-diluted Potts model

The short-time dynamics of the three-dimensional bond-diluted 4-state Potts model is investigated with Monte Carlo simulations. A recently suggested nonequilibrium reweighting method is applied, and the tricritical point is determined with the short-time dynamic approach. Based on the dynamic scaling form, both the dynamic and static critical exponents are estimated for the second order phase transition. Dynamic corrections to scaling are carefully considered.     

Monte Carlo investigation of the phase transition in the 2D Potts model with open boundary conditions

Finite size effects on the phase transition in the 2D Potts model with open boundary conditions are studied with Wang-Landau Monte Carlo simulations. We show the lattice size dependent cross-over from first order to continuous phase transition and discuss it in terms of surface induced disorder and size dependence of the latent heat.     

Transition from a two-dimensional superfluid to a one-dimensional Mott insulator

A two-dimensional system of atoms in an anisotropic optical lattice is studied theoretically. If the system is finite in one direction, it is shown to exhibit a transition between a two-dimensional superfluid and a one-dimensional Mott insulating chain of superfluid tubes. Monte Carlo simulations are consistent with the expectation that the phase transition is of Kosterlitz-Thouless type. The effect of the transition on experimental time-of-flight images is discussed.     

On the Ising model for the simple cubic lattice

The Ising model was introduced in 1920 to describe a uniaxial system of magnetic moments, localized on a lattice, interacting via nearest-neighbour exchange interaction. It is the generic model for a continuous phase transition and arguably the most studied model in theoretical physics. Since it was solved for a two-dimensional lattice by Onsager in 1944, thereby representing one of the very few exactly solvable models in dimensions higher than one, it has served as a testing ground for new developments in analytic treatment and numerical algorithms. Only series expansions and numerical approaches, such as Monte Carlo simulations, are available in three dimensions. This review focuses on Monte Carlo simulation. We build upon a data set of unprecedented size. A great number of quantities of the model are estimated near the critical coupling. We present both a conventional analysis and an analysis in terms of a Puiseux series for the critical exponents. The former gives distinct values of the high- and low-temperature exponents; by means of the latter we can get these exponents to be equal at the cost of having true asymptotic behaviour being found only extremely close to the critical point. The consequences of this for simulations of lattice systems are discussed at length.     

Statistical and systematic errors in Monte Carlo sampling

We have studied the statistical and systematic errors which arise in Monte Carlo simulations and how the magnitude of these errors depends on the size of the system being examined when a fixed amount of computer time is used. We find that, depending on the degree of self-averaging exhibited by the quantities measured, the statistical errors can increase, decrease, or stay the same as the system size is increased. The systematic underestimation of response functions due to the finite number of measurements made is also studied. We develop a scaling formalism to describe the size dependence of these errors, as well as their dependence on the bin length (size of the statistical sample), both at and away from a phase transition. The formalism is tested using simulations of thed=3 Ising model at the infinite-lattice transition temperature. We show that for a 96×96×96 system noticeable systematic errors (systematic underestimation of response functions) are still present for total run lengths of 10⁶ Monte Carlo steps/site (MCS) with measurements taken at regular intervals of 10 MCS.This paper is dedicated to Jerry Percus on the occasion of his 65th birthday.     

Phase transition analysis in Z2 and U(1) lattice gauge theories

The transition region of Z₂ lattice gauge theory is investigated by inverting the strong coupling series of the average plaquette energy E_P(J). We find a clear evidence for a first-order transition and the existence of a metastable phase. In the U(1) case we confirm a second-order phase transition even if there is a little discrepancy on the critical point position as indicated by Monte Carlo simulations.