基于马尔可夫随机场的改进Metropolis混凝土CT的图像分割方法

提出一种基于马尔可夫随机场改进的Metropolis模拟退火算法.该算法中阈值α的选取优化了Metropolis算法.根据Beyes理论将图像分割问题转化为最大后验概率求取问题,并给出了参量预测算法.在实验中,将本文提出的算法与ICM、传统的Metropolis和Gibbs采样模拟退火算法进行比较.结果表明改进的Metropolis算法在图像分割效率和分割准确度上都有明显的提高.通过混凝土CT图像分割实验,改进Metropolis的应用能够较完整地反映出混凝土材料的内部结构和缺陷.     

基于马尔可夫随机场的改进Metropolis混凝土CT的图像分割方法

提出一种基于马尔可夫随机场改进的Metropolis模拟退火算法.该算法中阈值α的选取优化了Metropolis算法.根据Beyes理论将图像分割问题转化为最大后验概率求取问题,并给出了参量预测算法.在实验中,将本文提出的算法与ICM、传统的Metropolis和Gibbs采样模拟退火算法进行比较.结果表明改进的Metropolis算法在图像分割效率和分割准确度上都有明显的提高.通过混凝土CT图像分割实验,改进Metropolis的应用能够较完整地反映出混凝土材料的内部结构和缺陷.     

基于文氏改进谱的二维粗糙海面模型及其电磁散射研究

基于文氏改进功率谱和Donelan方向分布的实验结论,结合经典的Monte Carlo法提出了一种适用于不同水深、不同风浪成长阶段的二维粗糙海面模型.在经典的双尺度法计算海面后向散射的基础上,结合小斜率近似法的仿真结果,对电磁散射计算方法进行改进.通过与典型海杂波均值散射系数GIT模型的拟合结果进行对比,验证了文氏改进谱在中国近海海面模拟的适应性与改进后电磁散射计算方法的有效性.     

巨正则量子Monte Carlo方法和二维Hubbard模型的研究

用巨正则量子Monte Carlo方法,计算了二维单带Hubbard模型的局域磁矩、磁化率、交错磁化率和内能等物理量.结果表明,用巨正则量子Monte Carlo方法能够统一地研究Hubbard模型的关联强度从弱至强区域的性质,它是处理强关联多体系统的有效方法.     

应用Monte Carlo方法模拟爆轰产物状态方程的研究进展

 应用Monte Carlo方法模拟爆轰产物状态方程,区别于建立在球形分布分子势基础上的传统爆轰产物状态方程计算方法,从原子水平上直接模拟实际炸药的爆轰产物状态方程,克服了传统方法中混合产物状态方程不精确的缺点。综述了该方法的历史背景及发展现状。对模拟中宏观量的描述、统计平均的计算、势函数的选取及边界条件进行了详尽描述。     

含有高阶参量的生物组织光学漫反射的半经验解析模型

漫反射光谱技术对生物组织快速和无创测量具有非常重要的意义,但缺乏精确的关于漫反射的解析解限制了该技术有效的应用.本文根据光子迁移理论和二阶相似关系推导了半经验公式,然后利用Monte Carlo方法研究了二阶参量γ对半经验公式的影响.研究表明,当小孔径收集漫反射光时,反射率与γ成二次函数关系.与其他含有高阶参量γ的解析模型相比,本文提出的半经验解析模型具有简单的数学形式,这为生物组织光学参量测量和漫反射光谱应用提供了新的理论和技术支持.     

磁性纳米颗粒系统偶极相互作用的Monte Carlo研究

采用局域Monte Carlo方法模拟不同易轴分布的简单立方排列单分散单畴Fe纳米颗粒系统的ZFC-FC曲线及磁滞回线.结果表明：随着偶极相互作用的增强，系统的阻塞温度T_B逐渐增大，且ZFC曲线的峰变宽.说明偶极相互作用使得系统的有效能垒提高，分布宽度增加.研究FC曲线磁化强度的倒数与温度关系，发现偶极相互作用系统中存在反铁磁有序.系统的阻塞态及超顺磁态的磁滞回线表明，极低低温下，随着偶极相互作用的增强，系统的矫顽力和剩磁减小，偶极相互作用阻碍系统的磁化；系统处于超顺磁态，各向异性作用及偶极相互作用使得系统的磁化曲线偏离Langevin曲线且偶极相互作用展现出退磁相互作用效应.偶极相互作用增强，系统磁化曲线与Langevin曲线偏差量的最大值向低场移动.在偶极相互作用下，易轴与外场夹角为45°的磁性纳米颗粒系统的平均有效能垒和有效能垒分布宽度较易轴随机分布系统的大.     

Monte Carlo方法模拟光在生物组织中传播的新进展

Monte Carlo方法已被广泛用于模拟复杂的随机介质如生物组织中光的辐射传输.在生物光子应用中,早期的Monte Carlo模拟模型忽略了光在组织中传输的波动性,而用中性粒子光子包来模拟其传播过程.然而,许多光学诊断技术是基于光在组织中的偏振效应和多重散射的相干性来揭示组织的生理和病理信息,这就要涉及光辐射的波动性.本文阐述了用Monte Carlo方法模拟光在生物组织中传播的最新进展.     

无损辨识时间依赖型内热源的 Monte Carlo方法

本文提出一种无损辨识时间依赖型内热源的Monte-Carlo算法,并成功地对两类不同形式的内热源进行了重构,结果证明了该算法的有效性.本文方法很容易推广到复杂的三维情形,也可推广用于空间依赖型内热源的无损辨识.     

On the Swendsen-Wang dynamics. I. Exponential convergence to equilibrium

We present rigorous results on the exponential convergence to equilibrium for the Swendsen-Wang stochastic dynamics for thed-dimensional Ising ferromagnet with external magnetic fieldh in the thermodynamic limit. We consider various situations, mainly in the low-temperature regime, in which boundary conditions are homogeneous and parallel or opposite to the external field. In the latter case we relate directly the tunneling from the metastable phase to the stable one with the exponential convergence to equilibrium.     

AN IMPROVED EFFECTIVE POTENTIAL APPROACH TO DYNAMICAL SYMMETRY BREAKING

We have developed, in this paper, a method for evaluating effective potential for local composite fields based on loop expansion by first renormalizing the generating functional of connected Green's functions and then making the Leqendre transformation. A consistency condition reflecting the fact that no condensate effect occurs at tree level is proposed, which plays an important role in this method. The method is illustrated through the Gross-Neveu model. This method can be applied to a wider class of models than can the auxiliary field method.     

Critical dynamics of the three-dimensional Ising model: A Monte Carlo study

Extensive Monte Carlo simulations have been performed to analyze the dynamical behavior of the three-dimensional Ising model with local dynamics. We have studied the equilibrium correlation functions and the power spectral densities of odd and even observables. The exponential relaxation times have been calculated in the asymptotic one-exponential time region. We find that the critical exponentz=2.09 ±0.02 characterizes the algebraic divergence with lattice size for all observables. The influence of scaling corrections has been analyzed. We have determined integrated relaxation times as well. Their dynamical exponentz _int agrees withz for correlations of the magnetization and its absolute value, but it is different for energy correlations. We have applied a scaling method to analyze the behavior of the correlation functions. This method verifies excellent scaling behavior and yields a dynamical exponentz _scal which perfectly agrees withz.     

Interface motion in a two-dimensional Ising model with a field

We determine by Monte Carlo simulations the width of an interface between the stable phase and the metastable phase in a two-dimensional Ising model with a magnetic field, in the case of nonconversed order parameter (Glauber dynamics). At zero temperature, the width increases ast with–1/3, as predicted by earlier theories. As temperature increases, the value of the effective exponent that we measure decreases toward the value 1/4, which is the value in the absence of magnetic field.     

A Comparison Between Broad Histogram and Multicanonical Methods

We discuss the conceptual differences between the broad histogram (BHM) and reweighting methods in general, and particularly the so-called multicanonical (MUCA) approaches. The main difference is that BHM is based on microcanonical, fixed-energy averages which depend only on the good statistics taken inside each energy level. The detailed distribution of visits among different energy levels, determined by the particular dynamic rule one adopts, is irrelevant. Contrary to MUCA, where the results are extracted from the dynamic rule itself, within BHM any microcanonical dynamics could be adopted. As a numerical test, we have used both BHM and MUCA in order to obtain the spectral energy degeneracy of the Ising model in 4×4×4 and 32×32 lattices, for which exact results are known. We discuss why BHM gives more accurate results than MUCA, even using the same Markovian sequence of states. In addition, such an advantage increases for larger systems.     

Comment on “A Comparison Between Broad Histogram and Multicanonical Methods”

A recent paper by A. R. Lima, P. M. C. de Oliveira, and T. J. P. Penna [J. Stat. Phys. 99:691 (2000)], seems to contain at least two mistakes which deserve comment, one concerning the numerical data, the other being of a conceptual kind.     

Energy of Long-Lifetime Configurations in Zero-Temperature Dynamics

Using Monte Carlo method with zero-temperature dynamics, we investigate energy evolution of Ising spin configuration on a square lattice. The energies of some configurations exhibit long duration before those configurations reach the final state -- ground state or frozen stripe state. For ground-state dynamical realization, the duration occurs when the energy per spin is 4/L, where L is the lattice size. For stripe-state dynamical realization, the energy is slightly higher than 2/L when the duration appears in the last evolution stage. In addition, it is found that the average energy per spin in final state is approximately 2/3L.     

Critical behaviour of the ferromagnetic Ising model on a triangular lattice

We use a new updated algorithm scheme to investigate the critical behaviour of the two-dimensional ferromagnetic Ising model on a triangular lattice with the nearest neighbour interactions. The transition is examined by generating accurate data for lattices with L= 8, 10, 12, 15, 20, 25, 30, 40 and 50. The updated spin algorithm we employ has the advantages of both a Metropolis algorithm and a single-update method. Our study indicates that the transition is continuous at T_c= 3.6403({2}). A convincing finite-size scaling analysis of the model yields υ=0.9995(21), β / υ = 0.12400({17}), γ / υ = 1.75223(22), γ '/υ=1.7555(22), α/υ= 0.00077(420) (scaling) and α / υ = 0.0010(42) (hyperscaling). The present scheme yields more accurate estimates for all the critical exponents than the Monte Carlo method, and our estimates are shown to be in excellent agreement with their predicted values.     

Surface Critical Behavior of Two-Dimensional Dilute Ising Models

Ising models with nearest neighbor ferromagnetic random couplings on a square lattice with a (1, 1) surface are studied, using Monte Carlo techniques and a star-triangle transformation method. In particular, the critical exponent of the surface magnetization is found to be close to that of the perfect model, Β₁ = 1/2. The crossover from surface to bulk critical properties is discussed.     

Finite-size effects in surface tension. I. Fluctuating interfaces

We consider a two-dimensional Ising cylinder of circumferenceM and heightN, with a floating interface introduced by the appropriate boundary conditions. An exact analysis of the finite-size effects in surface tension is given and the scaling function for all temperatures is calculated. The results are compared with the Monte Carlo data of Mon and Jasnow.On leave from: Department of Theoretical Chemistry, Oxford University, Oxford, OX1 3UB, England.