Molecular dynamics simulations of self-diffusion coefficient and thermal conductivity of methane at low and moderate densities

This article demonstrates a highly accurate molecular dynamics (MD) simulation of thermal conductivity of methane using an ab initio intermolecular potential. The quantum effects of the vibrational contribution to thermal conductivity are more efficiently accounted for in the present MD model by an analytical correction term as compared to by the Monte Carlo method. The average deviations between the calculated thermal conductivity and the experimental data are 0.92% for dilute methane and 1.29% for methane at moderate densities, as compared to approximately 20% or more in existing MD calculations. The results demonstrate the importance of considering vibrational contribution to the thermal conductivity which is mainly through the self-diffusion process.     

Hard-wall potential function for transport properties of alkali metal vapors

This study demonstrates that the transport properties of alkali metals are determined principally by the repulsive wall of the pair interaction potential function. The (hard-wall) Lennard-Jones (LJ) (15-6) effective pair potential function is used to calculate the transport collision integrals. Accordingly, reduced collision integrals of K, Rb, and Cs metal vapors are obtained from the Chapman-Enskog solution of the Boltzmann equation. The law of corresponding states based on the experimental transport reduced collision integral is used to verify the validity of a LJ(15-6) hybrid potential in describing the transport properties. LJ(8.5-4) potential function and a simple thermodynamic argument with the input PVT data of liquid metals provide the required molecular potential parameters. Values of the predicted viscosity of monatomic alkali metal vapor are in agreement with typical experimental data with average absolute deviations of 2.97% for K in the range of 700-1500 K, 1.69% for Rb, and 1.75% for Cs in the range of 700-2000 K. In the same way, the values of predicted thermal conductivity are in agreement with experiment within 2.78%, 3.25%, and 3.63% for K, Rb, and Cs, respectively. The LJ(15-6) hybrid potential with a hard-wall repulsion character conclusively predicts the best transport properties of the three alkali metal vapors.     

Prediction of the transport properties of a polyatomic gas

An ab initio molecular potential model is employed in this paper to show its excellent predictability for the transport properties of a polyatomic gas from molecular dynamics simulations. A quantum mechanical treatment of molecular vibrational energies is included in the Green and Kubo integral formulas for the calculation of the thermal conductivity by the Metropolis Monte Carlo method. Using CO₂ gas as an example, the fluid transport properties in the temperature range of 300–1000 K are calculated without using any experimental data. The accuracy of the calculated transport properties is significantly improved by the present model, especially for the thermal conductivity. The average deviations of the calculated results from the experimental data for self-diffusion coefficient, shear viscosity, thermal conductivity are, respectively, 2.32%, 0.71% and 2.30%.     

Cutoff radius effect of isotropic periodic sum method for transport coefficients of Lennard-Jones liquid

Molecular dynamics simulations of a Lennard-Jones (LJ) liquid were applied to compare the isotropic periodic sum (IPS) method [X. Wu and B. R. Brooks, J. Chem. Phys. 122, 044107 (2005)], which can reduce the calculation cost of long-range interactions, such as the Lennard-Jones and Coulombic ones, with the cutoff method for the transport coefficients which includes the self-diffusion coefficient, bulk viscosity, and thermal conductivity. The self-diffusion coefficient, bulk viscosity, and thermal conductivity were estimated with reasonable accuracy if the cutoff distance of the LJ potential for the IPS method was greater than 3sigma. The IPS method is an effective technique for estimating the transport coefficients of the Lennard-Jones liquid in a homogeneous system.     

Transport properties of Mie(14,7) fluids: molecular dynamics simulation and theory

An extensive computer simulation study is presented for the self-diffusion coefficient, the shear viscosity, and the thermal conductivity of Mie(14,7) fluids. The time-correlation function formalism of Green-Kubo is utilized in conjunction with molecular dynamics (MD) simulations. In addition to molecular simulations, the results of a recent study [A. Eskandari Nasrabad, J. Chem. Phys. 128, 154514 (2008)] for the mean free volume are applied to calculate the self-diffusion coefficients within a free volume theory framework. A detailed comparison between the MD simulation and free volume theory results for the diffusion coefficient is given. The density fluctuation theory of shear viscosity is used to compute the shear viscosity and the results are compared to those from MD simulations. The density and temperature dependences of different time-correlation functions and transport coefficients are studied and discussed.     

Molecular simulation of the shear viscosity and the self-diffusion coefficient of mercury along the vapor-liquid coexistence curve

In earlier work [G. Raabe and R. J. Sadus, J. Chem. Phys. 119, 6691 (2003)] we reported that the combination of an accurate two-body ab initio potential with an empirically determined multibody contribution enables the prediction of the phase coexistence properties, the heats of vaporization, and the pair distribution functions of mercury with reasonable accuracy. In this work we present molecular dynamics simulation results for the shear viscosity and self-diffusion coefficient of mercury along the vapor-liquid coexistence curve using our empirical effective potential. The comparison with experiment and calculations based on a modified Enskog theory shows that our multibody contribution yields reliable predictions of the self-diffusion coefficient at all densities. Good results are also obtained for the shear viscosity of mercury at low to moderate densities. Increasing deviations between the simulation and experimental viscosity data at high densities suggest that not only a temperature-dependent but also a density-dependent multibody contribution is necessary to account for the effect of intermolecular interactions in liquid metals. An analysis of our simulation data near the critical point yields a critical exponent of beta = 0.39, which is identical to the value obtained from the analysis of the experimental saturation densities.     

Thermal conductivity of methane hydrate from experiment and molecular simulation

A single-sided transient plane source technique has been used to determine the thermal conductivity and thermal diffusivity of a compacted methane hydrate sample over the temperature range of 261.5-277.4 K and at gas-phase pressures ranging from 3.8 to 14.2 MPa. The average thermal conductivity, 0.68 +/- 0.01 W/(m K), and thermal diffusivity, 2.04 x 10(-7) +/- 0.04 x 10(-7) m2/s, values are, respectively, higher and lower than previously reported values. Equilibrium molecular dynamics (MD) simulations of methane hydrate have also been performed in the NPT ensemble to estimate the thermal conductivity for methane compositions ranging from 80 to 100% of the maximum theoretical occupation, at 276 K and at pressures ranging from 0.1 to 100 MPa. Calculations were performed with three rigid potential models for water, namely, SPC/E, TIP4P-Ew, and TIP4P-FQ, the last of which includes the effects of polarizability. The thermal conductivities predicted from MD simulations were in reasonable agreement with experimental results, ranging from about 0.52 to 0.77 W/(m K) for the different potential models with the polarizable water model giving the best agreement with experiments. The MD simulation method was validated by comparing calculated and experimental thermal conductivity values for ice and liquid water. The simulations were in reasonable agreement with experimental data. The simulations predict a slight increase in the thermal conductivity with decreasing methane occupation of the hydrate cages. The thermal conductivity was found to be essentially independent of pressure in both simulations and experiments. Our experimental and simulation thermal conductivity results provide data to help predict gas hydrate stability in sediments for the purposes of production or estimating methane release into the environment due to gradual warming.     

Molecular dynamics investigation into the structural features and transport properties of C60 in liquid argon

Molecular dynamics (MD) simulations were performed to investigate the structural features and transport properties of C60 in liquid argon. The results reveal that an organized structure shell of liquid argon is formed close to the surface of a C60 fullerene molecule, thereby changing the solid/liquid interfacial structure. Furthermore, the simulation indicates that the C60-liquid argon fluid becomes structurally more stable as the C60 molecule volume fraction and the temperature increase. The viscosity of the fluid increases significantly as the C60 molecule loading is increased, particularly at a lower temperature. The thermal conductivity enhancement of the fluid in the present simulations is anomalously an order of magnitude higher than the theoretical predictions from either the Maxwell or the Lu and Liu models, and is found to vary approximately linearly with the C60 molecule volume fraction. The increased thermal conductivity is attributed to the nature of heat conduction in C60 molecule suspensions and an organized structure at the solid/liquid interface.     

Optimization of the anisotropic united atoms intermolecular potential for n-alkanes: improvement of transport properties

The parameters of the anisotropic united atom (AUA) intermolecular potential for n-alkanes originally proposed by Toxvaerd [J. Chem. Phys. 93, 4290 (1990)] [AUA(3)] was optimized by Ungerer et al. [J. Chem. Phys. 112, 5499 (2000)] [AUA(4)] on the basis of equilibrium properties (vapor pressures, vaporization enthalpies, and liquid densities). In this work we analyze the influence of the torsion potential in the internal and collective dynamics of the AUA model. The modified potential [AUA(4m)] preserves all the intermolecular parameters and only explores an increment in the trans-gauche and gauche(+)-gauche(-) transition barrier of the torsion potential. This modification better reproduce different transport properties (shear viscosity, self-diffusion coefficient, and internal relaxation times), keeping the accuracy achieved in our previous work for equilibrium properties. An extensive investigation of the shear viscosity of ethane, n-pentane, n-dodecane, and n-eicosane in a wide range of pressures and temperatures shows that the AUA(4m) improves the accuracy of the original AUA(4), reducing the absolute average deviation from 30% to 14.5%. Finally, the self-diffusion coefficient of n-hexane computed with the new model in the range of 223-333 K and from 0.1 to 295 MPa is in better agreement with respect to the experimental data than the original model.     

Investigating the Calculation of Rotational Viscosity of the Mixture Comprising Different Kinds of Liquid Crystals:Molecular Dynamics Computer Simulation Approach

Molecular dynamics (MD) computer simulation techniques, as a powerful tool commonly utilized by the liquid crystal display (LCD) community, usually are employed for computing the equilibrium and transport properties of a classical many body system, since they are very similar to real experiments in many respects. In this paper we present molecular dynamics computer simulation results taken for a mixture of the two different kinds of nematic liquid crystals (LCs). We calculated rotational viscosity from Brownian behavior with friction of the mean director of the mixture comprising pentylcyanobiphenol (5CB) and decylcyanobiphenol (10CB) by using molecular dynamics computer simulation, where intermolecular potential parameter is Generalized AMBER force field (GAFF). Our computed results show a good agreement with the experimental results.     

Dynamics and density profile of water in nanotubes as one-dimensional fluid

The transport and structural properties of water confined in nanotubes with different diameters were studied by molecular dynamics (MD) simulation. The effects of pore size, molecule-wall interaction, and the helicity of CNT on the diffusivity, thermal conductivity, and shear viscosity as well as density profile were analyzed. For diffusivity, in model NT > in armchair CNT > in zigzag CNT at similar conditions. However in contrast to the diffusivity, the thermal conductivity and the shear viscosity increase as the pore size decreases, in zigzag CNT > in armchair CNT > (or approximately ) in model NT. The ordered layer distribution of water molecules in nanotubes is clear. It suggests the structure of fluid in the zigzag CNTs is more ordered, and more solidlike. In the nanotubes, where the molecule and the pore dimensions are of similar order of magnitude, the nature of water-water and water-wall interactions, the confinement effect of space, and the helicity of CNT become more significant.     

Theoretical scheme for the shear viscosity of Lennard-Jones fluids

We use the shear viscosity expression from the Enskog theory of dense gases in a perturbative scheme for the Lennard-Jones (LJ) fluid. This perturbative scheme is formulated by combining the analytic rational function approximation method of Bravo Yuste and Santos [Phys. Rev. A 43, 5418 (1991)] for the radial distribution function of hard-sphere fluids and the well known Mansoori-Canfield/Rasaiah-Stell perturbation theory to determine an effective diameter for the LJ fluid. The scheme is reliable on a wide range of temperatures and densities, and is very accurate around the critical point. Using this information, we build an accurate empirical formula for the shear viscosity in the liquid phase, which fits the recent data [K. Meier et al., J. Chem. Phys. 121, 3671 (2004)] in the whole simulation range.     

Thermal conductivity of neon in the temperature range 400–2400K

Accurate measurements of the thermal conductivity of neon are given in the temperature range 400–2400K as obtained on an improved thermal conductivity column instrument. These data are found to be in good agreement with the earlier measurements of thermal conductivity from this laboratory and also with the available viscosity values. The present conductivity data are also compared with the predictions of kinetic theory in conjunction with several different forms for the intermolecular potential.     

Transport properties of the ionic liquid 1-ethyl-3-methylimidazolium chloride from equilibrium molecular dynamics simulation. The effect of temperature

We present here equilibrium molecular dynamics simulation results for self-diffusion coefficients, shear viscosity, and electrical conductivity in a model ionic liquid (1-ethyl-3-methylimidazolium chloride) at different temperatures. The Green-Kubo relations were employed to evaluate the transport coefficients. When compared with available experimental data, the model underestimates the conductivity and self-diffusion, whereas the viscosity is overpredicted, showing only a semiquantitative agreement with experimental data. These discrepancies are explained on the basis of the rigidity and lack of polarizability of the model. Despite this, the experimental trends with temperature are remarkably well reproduced, with a good agreement on the activation energies when available. No significant deviations from the Nernst-Einstein relation can be assessed on the basis of the statistical uncertainty of the simulations, although the comparison between the electric current and the velocity autocorrelation functions suggests some degree of cross-correlation among ions in a short time scale. The simulations reproduce remarkably well the slope of the Walden plots obtained from experimental data of 1-ethyl-3-methylimidazolium chloride, confirming that temperature does not alter appreciably the extent of ion pairing.     

Cause and effect reversed in non-equilibrium molecular dynamics: an easy route to transport coefficients

A novel non-equilibrium method for calculating transport coefficients is presented. It reverses the experimental cause-and-effect picture, e.g. for the calculation of viscosities: the effect, the momentum flux or stress, is imposed, whereas the cause, the velocity gradient or shear rates, is obtained from the simulation. It differs from other Norton-ensemble methods by the way, in which the steady-state fluxes are maintained. This method involves a simple exchange of particle momenta, which is easy to implement and to analyse. Moreover, it can be made to conserve the total energy as well as the total linear momentum, so no thermostatting is needed. The resulting raw data are robust and rapidly converging. The method is tested on the calculation of the shear viscosity, the thermal conductivity and the Soret coefficient (thermal diffusion) for the Lennard–Jones (LJ) fluid near its triple point. Possible applications to other transport coefficients and more complicated systems are discussed.     

Centroid molecular dynamics approach to the transport properties of liquid para-hydrogen over the wide temperature range

Fundamental transport properties of liquid para-hydrogen (p-H(2)), i.e., diffusion coefficients, thermal conductivity, shear viscosity, and bulk viscosity, have been evaluated by means of the path integral centroid molecular dynamics (CMD) calculations. These transport properties have been obtained over the wide temperature range, 14-32 K. Calculated values of the diffusion coefficients and the shear viscosity are in good agreement with the experimental values at all the investigated temperatures. Although a relatively large deviation is found for the thermal conductivity, the calculated values are less than three times the amount of the experimental values at any temperature. On the other hand, the classical molecular dynamics has led all the transport properties to much larger deviation. For the bulk viscosity of liquid p-H(2), which was never known from experiments, the present CMD has given a clear temperature dependence. In addition, from the comparison based on the principle of corresponding states, it has been shown that the marked deviation of the transport properties of liquid p-H(2) from the feature which is expected from the molecular parameters is due to the quantum effect.     

传递性质的自由体积模型对方阱流体的应用

对简单分子构成的稀薄气体的传递性质已有较严格的分子动力学理论，但对稠密流体，尤其是液体，严格的理论方法尚难以解决实际问题词．实用中仍以经验或半经验模型为主．半经验模型以Eyring的绝对反应速率理论[1，2]和自由体积理论[3-11]为典型代表．由于自由体积模型可以同时反映温度和压力的影响，其应用潜力更大，值得进一步研究．对理论模型的检验一般用实际物系的测定数据．这种检验虽然直接可靠，但往往受到实验条件的限制，例如高温高压；近年来迅速发展的计算机分子模拟为建立和验证理论模型提供了一个有效手段．由于计算机模拟数…     

Thermodynamic and transport properties of carbon dioxide from molecular simulation

Monte Carlo and molecular dynamics simulations have been used in order to test the ability of a three center intermolecular potential for carbon dioxide to reproduce literature experimental thermophysical values. In particular, both the shear viscosity under supercritical conditions and along the phase coexistence line, as well as the thermal conductivity under supercritical conditions, have been calculated. Together with the already reported excellent agreement for the phase coexistence densities, the authors find that the agreement with experimental values is, in general, good, except for the thermal conductivity at low density. Although extended versions of the model were employed, which include an explicit account of bending and vibrational degrees of freedom, a significant difference was still found with respect to the reported experimental value.