First-principles study of solute diffusion in Ni_3Al

Using first-principles calculations in combination with Wagner–Schottky and kinetic Monte Carlo methods, the diffusion behaviors of solutes via various vacancy-mediated diffusion mechanisms in L1_2 γ-Ni_3Al were investigated. The formation energies of the point defects and the migration energies for solutes were calculated. Adding alloying elements can decrease the defect-formation energies of Ni_(Al), increase the defect-formation energies of Al_(Ni), and have little effect on the formation energy of V_(Ni). The migration energies of solutes are related with the site preference and the diffusion mechanism. The diffusion coefficients of Ni, Al, and solutes were calculated, and the concentration of antisite defects plays a crucial role in the elemental diffusion.     

Modelling in relation to cation ordering

We review the methodology of using computer models to obtain quantitative information about cation ordering. Empirical interatomic potentials or ab initio electronic structure calculations are used to generate the energies for many configurations containing disordered arrangements of cations, and the parameters in model Hamiltonians can be determined from these energies. Monte Carlo simulations are then used to generate ensemble averages as functions of temperature or chemical composition. Analysis of the Monte Carlo ensembles directly yields the temperature dependence of long-range and short-range order, and thermodynamic quantities such as energy and heat capacity. Use of thermodynamic integration allows for the calculation of entropy and free energy. The methods are illustrated by examples showing long-range order/disorder phase transitions (feldspars), short-range order in solid solutions (pyrope-grossular), and non-convergent ordering (magnesium aluminate spinel); where comparisons with experimental data are possible, the model calculations are seen to give results that are reasonably accurate. The example in which ab initio electronic structure calculations are used show that it is now possible to extract accurate thermodynamic data for ordering processes using models that require no prior experimental data.     

Self-diffusion and ‘order–order’ kinetics in B2-ordering AB binary systems with a tendency for triple-defect formation: Monte Carlo simulation

Self-diffusion of component atoms and ‘order–order’ relaxations in a B2-ordering binary system AB showing a tendency for triple-defect formation were consistently simulated by means of two Monte Carlo techniques. In view of a strict correlation between antisite-defect and vacancy concentrations the Kinetic Monte Carlo (KMC) simulations were implemented with a temperature-dependent vacancy concentration determined by means of Semi-Grand Canonical Monte Carlo (SGCMC) simulations. The Ising model of the system was completed with local-configuration-dependent saddle-point energy parameters related to vacancy mediated atomic jumps. The simulations elucidated the atomistic origin of the experimentally observed low rate of ‘order–order’ relaxations in NiAl, as well as reproduced the experimental relation between the activation energies for ‘order–order’ kinetics and Ni self-diffusion in NiAl. Higher value of the deduced activation energy for atomic migration with respect to the effective energy barriers related to individual atomic jumps indicated their high correlation.     

Probing the structure and energetics of dislocation cores in SiGe alloys through Monte Carlo simulations

We present a methodology for the investigation of dislocation energetics in segregated alloys based on Monte Carlo simulations which equilibrate the topology and composition of the dislocation core and its surroundings. An environment-dependent partitioning of the system total energy into atomic contributions allows us to link the atomistic picture to continuum elasticity theory. The method is applied to extract core energies and radii of 60 degrees glide dislocations in segregated SiGe alloys which are inaccessible by other methods.     

On the correlation of crystal defects and band gap properties of ZnO nanobelts

The energetics and dynamics of electromigration of the oxygen vacancy is investigated with first-principles calculations and kinetic Monte Carlo methods. To simulate the charged oxygen vacancy under external fields within the first-principles approach, we introduce a slab model with electron-accepting dopants in the surface. The analysis of the density of states confirms that the oxygen vacancies are positively charged. When the external field is applied, the total energy of the slab linearly changes with respect to the position of the charged vacancy in the field direction, which allows for probing local permittivity around the vacancy site. The activation energy of vacancy migration is lowered along the field direction in a manner that the charge state of the vacancy is maintained along the migration path. Kinetic Monte Carlo simulations based on the first-principles inputs are also carried out and it is shown that the high-temperature condition is important for the fast redistribution of charged vacancies.     

基于变分Monte Carno计算的H2分子势能曲线的可靠性

基于Alexander and Coldwell报道的变分Monte Carno计算的势能曲线,计算了H2分子的振转能级.利用Murrell-Sorbie函数和最小二乘法拟合出了解析势能函数,以此为基础计算出光谱常数.通过与实验比较发现:由于该势能曲线考虑了相对论效应和非绝热效应,振转能级和光谱常数计算结果与实验值符合良好,与目前最高水平从头计算的结果相当,说明这种计算方法准确可靠,可进一步推广应用于势能函数计算之中.     

Diffusion correlation effects of molybdenum and silicon in molybdenum disilicide

Diffusion data for both principal directions of silicon and molybdenum as well as germanium are briefly summarised. Analysis is performed of the defect formation energies (available from previous ab initio calculations and experimental measurements) for diffusion mechanisms via home and foreign sublattices. The home sublattice mechanism is shown to be the preferred one for both silicon and molybdenum. Tracer correlation factors for silicon and molybdenum diffusion via sublattice vacancies in the respective sublattices of the tetragonal C11_b structure of molybdenum disilicide are calculated by a direct Monte Carlo simulation technique. Correlation factors for Si diffusion on its sublattice are compared with literature values that were calculated using a more complicated Monte Carlo method based on the matrix approach. It is shown that there is no need for this complicated approach and that the direct Monte Carlo simulation technique gives highly accurate correlation factors. Correlation factors and anisotropy ratios of vacancy-mediated diffusion in both sublattices are deduced and compared with experimental data. Tracer correlation in the tetragonal direction is shown to contribute 0.40?eV (i.e. over 55%) of the migration energy of the corresponding Si diffusivity. Two possible jump rates for Si diffusion are separately estimated. Mo diffusion correlation factors are calculated using the direct Monte Carlo technique. A comparison with experiment is made and the ratio of two possible jump rates is also estimated.     

Saddle-point energies and Monte Carlo simulation of the long-range order relaxation in CoPt

We present atomic-scale computer simulations in equiatomic L1₀-CoPt where Molecular Dynamics and Monte Carlo techniques have both been applied to study the vacancy-atom exchange and kinetics relaxation. The atomic potential is determined using a Tight-Binding formalism within the Second-Moment Approximation. It is used to evaluate the different saddle-point energies involved in a vacancy-atom exchange between nearest-neighbour sites. The potential and the saddle-point energies have been used to simulate the relaxation of the long-range order in CoPt using a Monte Carlo technique. A vacancy migration energy of 0.73±0.15 eV and an order-disorder transition temperature of 935 K have been found.     

低场低能区金刚石内载流子飞行时间的蒙特卡洛模拟

金刚石是一种重要的宽禁带半导体材料,对金刚石内载流子输运过程的研究将有助于了解金刚石用作各种电子器件的潜能。利用Monte Carlo模拟方法,研究了在低场低能区金刚石内载流子的飞行时间。在模拟中考虑了抛物线型能带模型和声学声子散射机制,以及样品对光的吸收和载流子在Brillouin区边界的Bragg反射。通过模拟,得到了低场低能区金刚石材料内载流子的飞行时间分布,并与相关的实验结果进行了比较分析,验证了该模拟模型的正确性。研究结果表明,在低场低能区,金刚石材料内主要的散射机制是声学声子散射。在研究金刚石材料内载流子的迁移输运问题时,可以采用较为简单的抛物线型能带模型,但在研究薄样品中的载流子输运时应当考虑材料的光吸收对初始载流子分布的影响,而且在场强较高以及样品厚度较大时,应当考虑载流子在Brillouin区边界的Bragg反射。     

Constrained path calculations of the <Superscript>4</Superscript><Emphasis Type="Italic">He</Emphasis> and <Superscript>16</Superscript><Emphasis Type="Italic">O</Emphasis> nuclei

We give results for the energy of the ⁴He and ¹⁶O nuclei using the auxiliary field diffusion Monte Carlo and a path constraint. We compare the results with previous FHNC and cluster Monte Carlo calculations.Received: 1 November 2002, Published online: 15 July 2003PACS: 21.10.Dr Binding energies and masses - 21.60.Ka Monte Carlo models     

Biexciton stability in carbon nanotubes

We have applied the quantum Monte Carlo method and tight-binding modeling to calculate the binding energy of biexcitons in semiconductor carbon nanotubes for a wide range of diameters and chiralities. For typical nanotube diameters we find that biexciton binding energies are much larger than previously predicted from variational methods, which easily brings the biexciton binding energy above the room temperature threshold.     

Ep≤63 GeV 质子入射铅、铋引起的中子产生双微分截面的模拟(英文)

加速器驱动次临界系统(ADS) 液态Pb-Bi 散裂靶的设计中,需要可靠的理论计算工具精确地预言几个GeV 能量范围的质子引起的散裂反应产生的各种粒子和核素。利用蒙特卡罗模拟软件包Geant4 计算研究了800 MeV至3 GeV 质子入射铅、铋材料引起的中子产生双微分截面。比较了Geant4 不同物理模型得到的模拟结果与现有的实验数据。其中,Geant4 的QGSP BERT和QGSP INCL ABLA 物理模型模拟结果很好地再现了实验数据。本工作证实了Geant4 蒙特卡罗模拟软件包适合用于能量高达3 GeV 的质子入射铅、铋引起的中子产生双微分截面的模拟计算。A detailed design of the liquid Pb-Bi spallation target of the Accelerator Driven Systems (ADS) requires powerful and reliable computational tools that can accurately predict particles and nuclides production by the proton induced spallation reactions in the energy range of a few GeV. In this paper, the neutron production double-differential cross sections for Pb and Bi target materials at incident proton kinetic energies between 800 MeV and 3 GeV are studied by calculations with Monte Carlo simulation package Geant4. The simulated results of Geant4 with several physics models are compared with available experimental data. The simulated results generated by QGSP BERT and QGSP INCL ABLA physics models of Geant4 well reproduce the available experimental data. The present results validated that Geant4 Monte Carlo simulation package is suitable for simulations of neutron production double-differential cross sections of proton induced reaction on Pb and Bi targets in the incident energy range up to 3 GeV.     

TheXY model and the three-state antiferromagnetic Potts model in three dimensions: Critical properties from fluctuating boundary conditions

We present the results of a Monte Carlo study of the three-dimensionalXY model and the three-dimensional antiferromagnetic three-state Potts model. In both cases we compute the difference of the free energies of a system with periodic and a system with antiperiodic boundary conditions in a neighborhood of the critical coupling. From the finite-size scaling behaviour of this quantity we extract values for the critical temperature and the critical exponentv that are compatible with recent high-statistics Monte Carlo studies of the models. The results for the free energy difference at the critical temperature and for the exponentv confirm that both models belong to the same universality class.     

Monte Carlo methods for yrast spectroscopy

We discuss the details of the recently proposed Monte Carlo method to evaluate the exact energies of yrast levels. Energy levels are evaluated up to J = 18 with small statistical errors using the Metropolis method for the case of ¹⁶⁶Er using the pairing plus quadrupole model within one major shell. We also discuss the evaluation of the probabilities of the Hartree-Fock-Bogoliubov wave functions in the corresponding yrast eigenstates and they are found to be large. The model displays a too strong backbending behaviour not seen experimentally.Received: 29 September 2003, Published online: 24 August 2004PACS: 21.60.-n Nuclear-structure models and methods - 02.70.Ss Quantum Monte Carlo methods - 21.60.Ka Monte Carlo models - 21.10.Re Collective levels     

Theoretical study of keto–enol tautomerism by quantum mechanical calculations (the QM/MC/FEP method)

In this study, the tautomeric equilibrium between the keto and enol forms has been studied for five typical ketones and aldehydes: i‐butanal, acetaldehyde, acetone, acetylacetone, and dimedone. The level of theory used in the gas‐phase calculation was Becke, three‐parameter, Lee–Yang–Parr/6‐311G(d,p)//Becke, three‐parameter, Lee–Yang–Parr/6‐31G(d). The free energies of solvation were included in the calculation by using the free‐energy perturbation method based on Monte Carlo simulation, that is, the quantum mechanical/Monte Carlo/free‐energy perturbation method. Three different models, incorporating no‐water, one‐water, and two‐waters, were adopted. The results showed that in the gas phase the addition of water molecules to the reaction mechanism caused the activation barriers (ΔG^?_gas) to decrease by half relative to the water‐free mechanism, but there was no effect on the relative difference in free energy, ΔG_gas. The solvation effects (ΔG_sol), based on quantum mechanical/Monte Carlo/free‐energy perturbation calculations, were added to those of the gas‐phase results of the one‐water and two‐waters models. The two‐waters model produced values that were very consistent with the experimental data for all of the tautomers. The differences in the relative Gibbs free energy (ΔG_rxn) were less than 1.0 kcal mol^–1. In summary, the inclusion of solvent molecules in gas‐phase calculations plays a very important role in producing results consistent with experimental data. Copyright © 2012 John Wiley & Sons, Ltd.     

An adaptive emission model for Monte Carlo simulations in highly inhomogeneous media represented by stochastic particle fields

Traditional Monte Carlo ray-tracing (MCRT) methods for continuous participating media are not applicable in media represented by point masses (or stochastic particles) frequently encountered in combustion modeling. In the authors’ previous work several ray models and particle models have been proposed for radiation simulations in such media. In the present paper an efficient emission scheme is developed for MCRT in highly inhomogeneous media represented by particle fields. Ray energies are limited to a narrow range to reduce statistical error, by having particles emit numbers of photons proportional to their emissive power (including combination of weak particles). A method to evaluate the radiative heat source, required by the overall energy equation, is also developed. A particle field representing the highly inhomogeneous medium in a turbulent jet flame is employed to test the proposed methods.     

宇宙线高能族事例产生特征的模拟与实验研究

利用在EAS实验的模拟计算中被广泛采用的CORSIKA程序,基于强相互作用模型QGSJET和DPMJET,对甘巴拉山乳胶室实验高能族事例的产生特征进行了Monte Carlo模拟.给出了不同能区原初宇宙线各种成分产生族事例的效率及其与原初能量的关系.将模拟计算得到的族事例的平均横向扩展、族中簇射成员数、族的总观测能等的分布及其平均值,与相应的实验结果进行了系统的比较,结果基本相符.此外,模拟族事例也存在与实验相类似的能量集中趋势     

Application of random walk concept to the cyclic diffusion mechanisms for self-diffusion in intermetallic compounds

Huntington–Elcock–McCombie (HEM) mechanism involving six consecutive and correlated jumps, a triple-defect mechanism (TDM) involving three correlated jumps and an anti-structure bridge (ASB) mechanism invoking the migration of an anti-structure atom are the three mechanisms currently in vogue to explain the self- and solute diffusion in intermetallic compounds. Among them, HEM and TDM are cyclic in nature. The HEM and TDM constitute the theme of the present article. The concept of random walk is applied to them and appropriate expressions for the diffusion coefficient are derived. These equations are then employed to estimate activation energies for self-diffusion via HEM and TDM processes and compared with the available experimental data on activation energy for self-diffusion in intermetallic compounds. The resulting activation energies do not favour HEM and TDM for the self-diffusion in intermetallic compounds. A comparison of the sum of experimentally determined activation energies for vacancy formation and migration with the activation energies for self-diffusion determined from radioactive tracer method favours the conventional monovacancy-mediated process for self-diffusion in intermetallic compounds.     

Solute-atom segregation at symmetric twist and tilt boundaries in binary metallic alloys on an atomic-scale

Monte Carlo and overlapping distributions Monte Carlo (ODMC) techniques are employed to simulate grain boundary (GB) segregation in a number of single-phase binary metallic alloys—the Au-Pt, Cu-Ni, Ni-Pd, and Ni-Pt systems. For a series of symmetric [001] twist and [001] tilt boundaries, with coincident site lattice (CSL) structures, we demonstrate that the Gibbsian interfacial excess of solute is a systematic function of the misorientation angle. We also explore in detail whether the GB solid solution behavior is ideal or nonideal by comparing the results of Monte Carlo and ODMC simulations. The range of binding free energies of specific atomic sites at GBs for solute atoms is also studied. The simulational results obtained demonstrate that the thermodynamic and statistical thermodynamic models commonly used to explain GB segregation are too simple to account for the microscopic segregation patterns observed, and that it is extremely difficult. If not impossible, to extract the observed microscopic information employing macroscopic models.