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1.
《Current Applied Physics》2015,15(9):1070-1074
Environmental problems due to the discharge of gases, including NO and CO2, in addition, diseases caused by improper concentration of NO and CO2 in vivo must be resolved. In this study, Grand canonical Monte Carlo (GCMC) simulations are combined with density functional theory (DFT) to calculate the adsorption of NO and CO2 from a dual-component mixture to the Cu-BTC metal organic framework. The results show that the adsorption isotherms for various molar ratios of the gaseous mixture followed a Langmuir distribution. At higher pressures more CO2 than NO was adsorbed by Cu-BTC, with NO showing a tendency to desorb. However, better results for adsorption of NO were observed at lower pressures. For the different pressure and molar ratios of the gaseous mixture examined, more CO2 than NO was always adsorbed. Compared with three-way catalysts, Cu-BTC offers benefits to adsorption of CO2 and NO from gaseous mixtures without increased durability problems.  相似文献   

2.
To deduce the optical properties, the absorption coefficient SmUaand reduced scattering coefficient μ’s, of turbid medium, Lin et al. (Appl. Opt. 34 (1995) 2362) proposed an oblique incidence reflectometry in which the diffusion approximation was assumed. In this paper we propose an alternative method which does not assume the diffusion approximation but uses a Monte Carlo light propagation model. Two features are extracted from the diffuse reflectance distribution detected on the medium surface, and optical properties are then estimated by looking up the predetermined table generated by Monte Carlo simulations. The validity of the proposed method has been confirmed by computer simulations.  相似文献   

3.
We suggest and implement a new Monte Carlo strategy for correlated models involving fermions strongly coupled to classical degrees of freedom, with accurate handling of quenched disorder as well. Current methods iteratively diagonalise the full Hamiltonian for a system of N sites with computation time τN ∼N4. This limits achievable sizes to N ∼100. In our method the energy cost of a Monte Carlo update is computed from the Hamiltonian of a cluster, of size Nc, constructed around the reference site, and embedded in the larger system. As MC steps sweep over the system, the cluster Hamiltonian also moves, being reconstructed at each site where an update is attempted. In this method τN,Nc ∼NNc3. Our results are obviously exact when Nc=N, and converge quickly to this asymptote with increasing Nc, particularly in the presence of disorder. We provide detailed benchmarks on the Holstein model and the double exchange model. The `locality' of the energy cost, as evidenced by our results, suggests that several important but inaccessible problems can now be handled with control. This method forms the basis of our studies in Europhys. Lett. 68, 564 (2004), Phys. Rev. Lett. 94, 136601 (2005), and Phys. Rev. Lett. 96, 016602 (2006).  相似文献   

4.
J. Largo 《Molecular physics》2016,114(16-17):2391-2399
ABSTRACT

The thermodynamic properties of a fluid with an interaction potential consisting in a hard-sphere core plus a Lennard-Jones tail have been obtained by Monte Carlo (MC) NVT simulation as a function of the density along several isotherms. In addition, the liquid–vapour coexistence has been determined by means of histogram-reweighting MC. These data have been used to analyse the performance of perturbation theory. To this end, the first three perturbation terms of the inverse temperature expansion of the Helmholtz free energy have been obtained by means of MC NVT simulations to test the convergence of the perturbation series and to compare with the predictions of the coupling parameter series expansion. Then, the predictions of the latter theory for the thermodynamic properties have been compared with the simulations, revealing the overall excellent performance of this perturbation theory for this model fluid, except in the vicinity of the critical point.  相似文献   

5.
Rotational-isomeric-state (RIS)-Metropolis Monte Carlo simulations are performed on poly(styrene-ran-methylmethacrylate) random copolymers to study the intrinsic unperturbed (θ-condition) dimensions. Mean-squared end-to-end distance (?r2?o ), mean-squared radius of gyration (?s2?o ), and characteristic ratio (Cn) have been calculated for these copolymers constituted by different overall chemical compositions (styrene fractions 0.29, 0.56, and 0.70). Calculations were carried out with chains of 500 repeating units. With an increase in the styrene content there is an increase in ?r 2? o, `, and Cn, in agreement with experimental observations. An increase in the fraction of trans conformational states in the backbone torsion angles is found to be responsible for the exhibited chain expansion behavior. The dimensions calculated by the Monte Carlo simulations agree well with experimental values reported in the literature.  相似文献   

6.
F. CALVO 《Molecular physics》2013,111(21):3421-3427
The multiple range random walk algorithm recently proposed by Wang and Landau [2001, Phys. Rev. Lett., 86, 2050] is adapted to the computation of free energy profiles for molecular systems along reaction coordinates. More generally, we show how to extract partial averages in various statistical ensembles without invoking simulations with constraints, biasing potentials or unknown parameters. The method is illustrated on a model 10-dimensional potential energy surface, for which analytical results are obtained. It is then applied to the potential of mean force associated with the dihedral angle of the butane molecule in the gas phase and in carbon tetrachloride solvent. Finally, isomerization in a small rocksalt cluster, (NaF)4, is investigated in the microcanonical ensemble, and the results are compared to those of parallel tempering Monte Carlo.  相似文献   

7.
While conventional crystal structure analysis using Bragg intensities reveals only information about the average structure of the crystal, diffuse scattering contains additional information about the disorder, i.e. departure from the average structure, of the studied material. Two different approaches to the analysis of diffuse scattering based on Monte Carlo methods are described in this paper: the direct Monte Carlo (MC) simulation technique and the Reverse Monte Carlo (RMC) method. The MC method requires the construction of a model for the disorder based on physical and chemical considerations and the selection of a set of near-neighbour interactions. The given model is realized by minimizing the total energy of the crystal via MC simulations. Next, the corresponding diffraction pattern is calculated and compared with the experimental data. By adjusting the near-neighbour interaction and repeating the process, a qualitative “match” between observed and calculated diffuse scattering is obtained. In contrast, the RMC method minimizes the difference between observed and calculated diffuse scattering intensities directly. This method leads to one real space structure consistent with the observed diffuse scattering but does not automatically result in a chemically sensible structure or further insight into the particular disorder problem.

The first example given in this paper demonstrates the viability of the RMC method by refining diffuse scattering data calculated from simulated structures with known disorder parameters. These structures were generated using the MC technique. As a second example MC and RMC simulations of the diffuse scattering of stabilized zirconias (CSZ) are shown, modelling occupational disorder as well as displacements.  相似文献   

8.
Braden Kelly 《Molecular physics》2019,117(20):2778-2785
ABSTRACT

We describe a new algorithm for the molecular simulation of chemical reaction equilibria, which we call the Reactive Kinetic Monte Carlo (ReKMC) algorithm. It is based on the use of the equilibrium Kinetic Monte Carlo (eKMC) method (Ustinov et al., J. Colloid Interface Sci., 2012, 366, 216–223) to generate configurations in the underlying nonreacting system and to calculate the species chemical potentials at essentially zero marginal computational cost. We consider in detail the typical case of specified temperature, T and pressure, P, but extensions to other thermodynamic constraints are straightforward in principle. In the course of this work, we also demonstrate an alternative method for calculating simulation box volume changes in NPT ensemble simulations to achieve the specified P. We consider two sets of example reacting systems previously considered in the literature, and compare the ReKMC results and computational efficiencies with those of different implementations of the REMC algorithm (Turner et al., Molec. Simulation, 2008, 34, 119–146).  相似文献   

9.
H2+He流体混合物在高温高压下由于氢的离解化学反应形成由H2,H,He三种粒子构成的混合体系,此时粒子间的相互作用较为复杂,离解能也会由于粒子间的这种复杂相互作用而降低.本文利用自洽流体变分理论来研究部分离解区H2+He流体混合物的高温高压物态方程,模型考虑了各种粒子间的相互作用及由温致和压致效应引起的离解能降低的自洽变分修正,并通过自洽流体变分过程对非理想的离解平衡方程求解得到粒子数密度分布,进而对自由能求导获得体系的热力学状态参量.计算结果与已有的冲击波实验数据、蒙特卡罗模拟及其他理论计算进行了比较.  相似文献   

10.
We propose a method to calculate the temperature dependence of Heisenberg exchange coupling constants Jij. Within the formalism of disordered local moments (DLM), the magnetization and the Jij are computed from first principles for any concentration c of the magnetic constituents. The exchange coupling constants are then used in Monte Carlo (MC) simulations to compute the temperature dependence of the magnetization for the given c. By comparing the magnetization from DLM calculations and from MC simulations we obtain a mapping of temperature versus concentration and eventually temperature-dependent Jij. The approach which is applied to bulk Fe and Co can for example improve critical exponents.  相似文献   

11.
The Gibbs Ensemble Monte Carlo method has been used to simulate osmotic equilibria for Lennard-Jones mixtures. When the simulations are performed with two independent boxes, one containing solvent and the other a mixture of solute and solvent significantly negative osmotic pressures (Π) develop. Following a sugestion of Powles et al. (1997, Molec. Phys., 90, 665), we have extended these simulations to include a third box and the possibility of modelling three coexisting phases. The simulations show that the two phase equilibria with negative values of 77 are metastable and that the system spontaneously separates into three phases: pure solvent, dilute solute-solvent and dense solute-solvent with a resulting osmotic pressure that is normally small and positive.  相似文献   

12.
Abstract

Proton NMR relaxation measurements were performed for ammonium bromide at pressures up to 800 MPa. The correlation functions for ammonium ion reorientation were calculated by Monte Carlo method. Knowledge of these functions allow calculation of the NMR relaxation times T1. Comparison between experimental and calculated values of relaxation times gives some insight into the model of ammonium ion reorientation.  相似文献   

13.
14.
In the present work, we have used a simple equation of state called the GMA EoS to calculate the density of three ionic liquid mixtures including 1-butyl-3-methylimidazolum hexafluorophosphate, [BMIM] [PF6] + methanol, 1-butyl-3-methylimidazolum tetrafluoroborate, [BMIM] [BF4] + methanol, and [BMIM] [BF4] + ethanol at different temperatures, pressures, and compositions. The isothermal compressibility, excess molar volumes, and excess Gibbs molar energy of these mixtures have been computed using this equation of state. The values of statistical parameters show that the GMA EoS can predict these thermodynamic properties very well within the experimental errors. The results show that isothermal compressibility of ionic liquids is lower than alcohols and the effect of temperature and pressure on the isothermal compressibility of ionic liquids is lower than alcohols. The excess molar volumes and excess molar Gibbs energy for these ionic liquid mixtures with alcohols are all negative at various temperatures and pressures over the whole composition range. The results have been interpreted in terms of intermolecular interactions and structural factors of the ionic liquids and alcohols.  相似文献   

15.
The mechanism of the growth of the dendrites in the Ni-Cu films is studied by comparing them with the aggregates obtained by Monte Carlo (MC) simulations according to the diffusion-limited aggregation (DLA) model. The films were grown by electrodeposition. The structural analysis of the films carried out using the x-ray diffraction showed that the films have a face-centered cubic structure. Scanning electron microscope (SEM) was used for morphological observations and the film compositions were determined by energy dispersive x-ray spectroscopy. The observed SEM images are compared with the patterns obtained by MC simulations according to DLA model in which the sticking probability, P between the particles is used as a parameter. For all samples between the least and the densest aggregates in the films, the critical exponents of the density-density correlation functions, α were within the interval 0.160 ± 0.005-0.124 ± 0.006, and the fractal dimensions, Df, varies from 1.825 ± 0.006 to 1.809 ± 0.008 according to the method of two-point correlation function. These values are also verified by the mass-radius method. The pattern with α and Df within these intervals was obtained by MC simulations to DLA model while the sticking probability, P was within the interval from 0.35 to 0.40 obtained by varying P (1-0.001). The results showed that the DLA model in this binary system is a possible mechanism for the formation of the ramified pattern of Ni-Cu within the Ni-rich base part of the Ni-Cu films due to the diffusive characteristics of Cu.  相似文献   

16.
《Molecular physics》2012,110(11-12):1213-1221
Thermodynamic models based on the van der Waals–Platteeuw statistical theory (Adv. Chem. Phys. 2, 1 (1959)) can be very accurate in describing hydrate equilibrium conditions, even for some occasions when multiple cavity occupancy occurs. These are cases outside the range of assumptions used for the development of the original statistical theory. However, during multiple cavity occupancy such models can perform poorly when calculating the cavity occupancies. This paper reports novel Grand Canonical Monte Carlo molecular simulations for the case of pure structure II N2 hydrate and compares the calculated cavity occupancies with experimental data and observe reasonable agreement. Also examined are the van der Waals–Platteeuw-based modifications that retained the single-occupancy assumption of the original theory and how they perform when predicting cavity occupancies and hydrate equilibrium pressures.  相似文献   

17.
The parameters of two pair potentials that describe argon over its entire liquid phase at a fixed pressure were optimized through a novel application of constant temperature and pressure molecular dynamics (NPT-MD) and Monte Carlo (NPT-MC) computer simulations. The forms of these potentials were those of a modified Lennard-Jones potential and a Lennard-Jones potential. The optimized potential determined using NPT-MD simulations reproduces experimental densities, internal energies and enthalpies with an error less than 1% over most of the liquid range and yields self-diffusion coefficients that are in excellent agreement with experiment. The results using the potential determined by NPT-MC simulations are in almost as good agreement with deviations from experiment of no more than 5.89% for temperatures up to vaporization. Additionally, molar volumes predicted using this potential at pressures in the range 100–600 atm and over temperatures in the range 100–140K were within 0.83% of experimental values. These results show that, when properly parametrized, Lennard-Jones-like potentials can describe a system well over a large temperature range. Further, the method introduced is easy to implement and is independent of the form of the interaction potential used.  相似文献   

18.
The equilibrium heat capacities of model pure and heterogeneous water clusters have been calculated using exchange Monte Carlo simulations. For the pure water cluster (H2O)20, microcanonical and canonical caloric curves obtained from various rigid intermolecular potentials indicate the onset of melting to lie in the range 140–180 K, in reasonable agreement with previous estimates. Clusters doped with a single hydronium or ammonium impurity show a significant shift of the melting point in the 20-molecule system, but a reduced effect when 50 molecules are reached.  相似文献   

19.
20.
Cutouts, which are used as field-shaping shield, affect several electron beam parameters. These effects are more observable for small field sizes and high energy electron beams. Owing to the fact that small fields prevent the lateral scatter equilibrium, at higher energies larger field radius is required for the establishment of lateral equilibrium.The profile curves are derived from circular, triangular, and square cutout shapes and size placed in a 10 × 10 cm2 electron applicator. These profile curves are obtained using parallel plane type ion chamber at the R100, R90, R80 and R50 depths. Correspondingly, the source surface distance is 100 cm.In this study MCNP Monte Carlo (MC) simulation was used to compare Percentage Depth Dose (PDD) and Profile of electron beams.Monte Carlo and measured results showed a good compliance for PDD and beam profile. The measurements and calculations showed that as the field width decreases, the Flatness and Penumbra Ratio also decreases. In other words, flatter plateau was available for larger fields. Also the Coverage Ratio for each of the profiles is presented. The flatness and symmetry values for triangle shapes were greater than the two other shapes.Knowledge of these changes are significant in radiation therapy. Accordingly, a comparison between the Monte Carlo data and the measured results can be beneficial for treatment simulation and development of treatment planning systems.  相似文献   

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