Isothermal-isobaric Molecular Dynamics Simulations Research Articles

Excess properties of ternary fluid mixtures from simulation, perturbation theory and van der Waals one-fluid theory: size and energy parameter effects

In this paper, we report isothermal-isobaric molecular dynamics simulation results for total and excess properties of 48 equimolar and 24 nonequimolar ternary mixtures consisting of Lennard-Jones fluids in different nonideal conditions. Energy and size parameters of the components differ appreciably, lying in the range ( 1 ≤ ε BB ε AA ≤ 1.75 , 1 ≤ ε CC ε AA ≤ 2 ) and ( 1 ≤ σ BB σ AA ≤ 1.75 , 1 ≤ σ CC σ AA ≤ 2 ). Unlike energy and size parameters of mixtures are determined using Lorentz-Berthelot combination rule. Mixtures are considered at four different conditions, ( kT ε AA = 1 , Pσ AA 3 ε AA = 0.5 ), ( kT ε AA = 2 , Pσ AA 3 ε AA = 1.2 ), ( kT ε AA = 2 , Pσ AA 3 ε AA = 2.5 ) and ( kT ε AA = 3 , Pσ AA 3 ε AA = 2.5 ). The effects of unlike size and energy interaction parameters on total and excess properties of mixtures deviating from the Lorentz-Berthelot combination rule have also been investigated at two different conditions, ( kT ε AA = 1 , Pσ AA 3 ε AA = 0.5 ) and ( kT ε AA = 2 , Pσ AA 3 ε AA = 2.5 ). These simulation results have been used to test our recently developed first-order perturbation theory and van der Waals one-fluid theory of ternary mixtures. Perturbation theory is based on the hard sphere reference mixture and involves no mixing rules. Van der Waals one-fluid theory is based on an accurate equation of state for the pure fluids. An extensive comparison of theoretical predictions with simulation results show that, in general, perturbation theory is very successful in describing simulation results for ( 1 ≤ ε BB ε AA ≤ 1.75 , 1 ≤ ε CC ε AA ≤ 2 ) and ( 1 ≤ σ BB σ AA ≤ 1.75 ; 1 ≤ σ CC σ AA ≤ 2 ). On the other hand, van der Waals one-fluid theory is reliable in describing total properties in the range 1 ≤ σ CC σ AA ≤ 1.18 and excess properties in the range 1 ≤ σ BB σ AA ≤ 1.25 , for all the energy parameter ratios considered in this paper.

Molecular Dynamics Simulation Study of the Behavior of Molecular Crystal Tin Tetraiodide under Hydrostatic Pressure

Isothermal-isobaric molecular dynamics simulations of molecular crystal SnI 4 are carried out based on a model consisting of rigid tetrahedral molecules interacting via van der Waals forces. The behavior of the crystal upon an increase in pressure is satisfactorily reproduced on both microscopic and macroscopic scales in the pressure regime in which the low-pressure phase is stable. In particular the compression curve obtained is in excellent agreement with the experimentally determined one. Upon further increase in pressure, however, no symmetry change is detected in the present simulations over the pressure regime in which the phase transformation and the subsequent solid state amorphization are experimentally found to take place.

Thermodynamic properties of binary mixtures of atoms and quadrupolar diatomic molecules from computer simulation

This paper reports results obtained from isothermal-isobaric molecular dynamics simulations of binary fluid mixtures composed of one-center Lennard-Jones atoms and two-center Lennard-Jones with point quadrupole molecules. These simulation results are important to investigate the effects of molecular elongation and quadrupole moment on excess properties and to test statistical theories of highly nonideal molecular fluid mixtures. Molecular size and energy parameters of the components are kept to be the same. We show how thermodynamic properties respond to changes in both molecular elongation and quadrupole moment. Simulations are performed over a range of molecular elongation 0 ≤ l d = L σ d ≤ 0.6 and quadrupole moment 0 ≤ q d = Q (ε a σ a 5) 1 2 ≤ 2.5 . Results are reported for mixture density, internal energy, excess Gibbs free energy, excess enthalpy and excess volume of 17 equimolar mixtures at a temperature, k b T ε a = 0.92 , and pressure, Pσ a 3 ε a = 0.5 . Excess Gibbs free energy has been determined accurately using the highly reliable coupling parameter charging method. It has been found that excess properties increase as the quadrupole moment increases for a fixed molecular elongation. When the both molecular elongation and quadrupole moment increase simultaneously, there is a strong competition between their effects on the excess properties.

Total and excess properties of nonideal ternary fluid mixtures via isothermal-isobaric molecular dynamics simulation: size and energy parameter effects

We report isothermal-isobaric molecular dynamics simulation results for total and excess properties of 162 ternary mixtures of Lennard-Jones fluids. Simulations were performed at energy and size parameter ratios in the range ( 1 ≤ ϵ CC ϵ AA ≤ 2 ; 1 ≤ σ CC σ AA ≤ 2 ). In these mixtures, the component A consists of the smallest atoms, while the component C consists of the biggest atoms. The energy and size parameter ratios of the component B lie in the range ( 1 ≤ ϵ BB ϵ AA ≤ 1.75 ; 1 ≤ σ BB σ AA ≤ 1.75 ). Results are reported for mixture density, residual internal energy, excess enthalpy and excess volume of 120 equimolar mixtures and 42 nonequimolar mixtures at 5 different thermodynamic conditions: ( kT ϵ AA = 1 , Pσ AA 3 ϵ AA = 0.5 ); ( kT ϵ AA = 1.5 , Pσ AA 3 ϵ AA = 1.5 ); ( kT ϵ AA = 2 , Pσ AA 3 ϵ AA = 1.2 ); ( kT ϵ AA = 2 , Pσ AA 3 ϵ AA = 2.5 ) and ( kT ϵ AA = 3 , Pσ AA 3 ϵ AA = 2.5 ). Some of these simulation results are used to test the accuracy of the recently developed statistical mechanical perturbation theory and van der Waals one-fluid theory of ternary mixtures. Perturbation theory is very successful in describing simulation results of highly nonideal mixtures, while van der Waals one-fluid theory can describe moderately nonideal mixtures only.

Thermodynamic properties of binary mixtures of atoms and diatomic molecules from computer simulation and perturbation theory

This paper reports results obtained from isothermal-isobaric molecular dynamics simulations of binary fluid mixtures composed of atoms and homonuclear diatomic molecules. Intermolecules. Intermolecular interactions between atoms are represented by the one-center Lennard-Jones potential and those between diatoms are represented by the two-center Lennard-Jones potential. Simulations are performed over a range of molecular elongation 0 ≤ I d = L/σ d ≤ 0.8. Molecular size and energy parameters of the components are kept the same. Results are reported for density, internal energy, excess Gibbs free energy, excess enthalpy and excess volume of several equimolar mixtures at a temperature, k b T/ε a = 0.92, and pressure Pσ a 3/ε a = 0.5. Excess Gibbs free energy has been determined accurately using the reliable coupling parameter charging method. We show how thermodynamic properties respond to changes in molecular elongation. Also, we have used our simulation results to test a new version of the first-order perturbation theory of two-center Lennard-Jones fluid mixtures based on the nonspherical reference system. In this theory, correlation functions of the reference fluid mixture are approximated by those of the hard sphere mixture. It has been found that this version of the perturbation theory can describe density and internal energy up to l d = 0.8, and it can predict simulation results for excess properties accurately for l d < 0.6.

Evidence of hexatic phase formation in two-dimensional Lennard-Jones binary arrays.

We report evidence of the hexatic phase formation in Lennard-Jones binary substitutional random arrays from isothermal-isobaric molecular-dynamics simulations. The hexatic phase is analogous to those predicted in Kosterlitz-Thouless theory of melting that is characterized by short-range translational order and quasi-long-range orientational order. At the crystal to hexatic phase transition, dislocation pairs are observed to unbind into isolated dislocations. Further disordering of the hexatic phase, however, does not lead to dissociation of dislocations into disclinations. Instead, the dislocations become clustered and form dislocation networks which results in formation of amorphous phases. {copyright} {ital 1996 The American Physical Society.}

Excess Thermodynamic Properties of Ternary Fluid Mixtures from Molecular Dynamics Simulation and Van Der Waals One-Fluid Theory

Abstract In this paper, we report isothermal–isobaric molecular dynamics simulation results for total and excess thermodynamic properties of three selected nonideal ternary fluid mixtures, in which particles interact via a Lennard-Jones pair potential. In these mixtures, size parameters of the components are kept to be the same, but energy parameters of the components differ significantly. Simulation results are reported for mixture density, internal energy, excess Gibbs free energy, excess volume and excess enthalpy of equimolar ternary mixtures as a function of temperature and pressure. The excess Gibbs free energy is obtained directly using the coupling parameter charging approach, which avoids the cancellation or magnification of errors in the simulation. These simulation results are used to check the accuracy of a form of the van der Waals one–fluid theory, which uses an accurate equation of state for the pure fluids. Van der Waals one–fluid theory is successful in predicting total and excess properties of these nonideal mixtures within simulation uncertainties.

Excess Thermodynamic Properties of CO2+ C2H6Mixtures from MD Simulations

Abstract This paper reports excess Gibbs free energy, excess enthalpy, and excess volume from isothermal-isobaric molecular dynamics simulations of carbon dioxide + ethane mixtures at T = 230 K, 260 K, P = 15.5 MPa, and x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9. The overall agreement of our calculations with experiments is excellent. The excess Gibbs free energies were calculated by two different methods. They showed that the energy difference method for swapping particles of CO2 into particles of C2H6 yields more reliable values.

Determination of Thermodynamic Excess Properties of Mixtures from Computer Simulation

Abstract This paper reports excess Gibbs free energy, excess enthalpy, and excess volume from isothermal-isobaric molecular dynamics simulations of binary Lennard-Jones mixtures at κ T/∊ = 2, pσ3/∊ = 2.5, for three parameter ratios, ∊ BB /∊ AA = 1 and σ BB /σ AA = 1.5, ∊ BB /∊ AA = 1.5 and σ BB /σ AA = 1, ∊ BB /∊ AA = 1.5 and σ BB /σ AA = 1.5, and at different concentrations, xA = 0.02, 0.05, 0.1, 0.25, 0.5, 0.75, 0.9, 0.95, and 0.98. The density range is between 0.25 < ρ∗ < 0.75. The excess Gibbs free energies were determined by fitting the differences in residual chemical potentials, which were evaluated by using the ‘fA – fB sampling’ method. The results showed very good agreement with literature data.

Structures of disordered alkali chlorides in normal and compressed states: An isothermal-isobaric molecular-dynamics study.

Isothermal-isobaric molecular-dynamics simulations have been performed to investigate the glassy and liquid structures of two alkali chloride systems [mixture (LiCl${)}_{0.50}$(KCl${)}_{0.40}$(CsCl${)}_{0.10}$ and pure LiCl]. With the use of an ionic interaction model, the basic thermodynamic properties of the crystalline, liquid, and glassy states are successfully reproduced in the simulations. At normal pressure (101.3 MPa), it is found that the predominant short-range order in both systems is the ${\mathrm{LiCl}}_{4}$ tetrahedral units, each pair of which is mutually connected by sharing not only the vertices but also the edges of the tetrahedra. In the glassy and liquid states of LiCl-KCl-CsCl, the network structure is formed by polytetrahedral medium-range order (${\mathrm{LiLi}}_{4}$) consisting of five connected ${\mathrm{LiCl}}_{4}$ tetrahedra. Some portions of this network are truncated by the ${\mathrm{K}}^{+}$ and ${\mathrm{Cs}}^{+}$ ions adjacent to the vertex ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ ions. On the other hand, liquid and glassy LiCl has a disordered structure approximately analogous to zinc-blende structure, also including the wurtzitelike ionic arrangement. For LiCl in the amorphous and crystalline states, we performed isothermal-compression simulations up to 30 GPa. As in the experimental findings, no structural transformation occurs for the rocksalt LiCl crystal. The compression causes the crystallization of amorphous LiCl into rocksalt structure at more than 3--10 GPa with continuous structural change. This structural transformation is analogous to the pressure-induced polymorphic transition (zinc blende\ensuremath{\rightarrow}rocksalt) that occurs in the crystals of more covalent compounds such as CdS.

Hoover NPT dynamics for systems varying in shape and size

In this paper we write down equations of motion (following the approach pioneered by Hoover) for an exact isothermal-isobaric molecular dynamics simulation, and we extend them to multiple thermostating rates, to a shape-varying cell and to molecular systems, coherently with the previous ‘extended system method’. An integration scheme is proposed together with a numerical illustration of the method.

Computation Confirms Contraction: A Molecular Dynamics Study of Liquid Methanol, Water and a Methanol-Water Mixture

Abstract It may be questioned whether potential models that have been developed independently for two different pure compounds would behave properly when used in computer simulations of mixtures of these compounds. Since they are optimized for the pure compounds there is no guarantee whatsoever that the terms describing the interaction between dissimilar molecules are correct. If the simulational and experimental values of several thermodynamical properties of the mixture relative to those of the pure compounds agree closely, however, this strongly indicates that no separate optimizations need be carried out for the mixtures. Here we present the results of isothermal-isobaric Molecular Dynamics simulations of liquid methanol, water and equimolar methanol-water mixtures, using simple point charge models. The potential parameters of the models for the pure liquids had been independently optimized. No adjustment of parameters was made for the mixture, but nonetheless the experimental volume contraction and e...

Isothermal–isobaric molecular dynamics simulation of liquid water

Results of the first isothermal–isobaric molecular dynamics simulation of water are reported. Water molecules are assumed to be flexible and to interact via a sum of pairwise interatomic potentials. Extensive runs for 256 molecules at 298 and 373 K and 1 bar give satisfactory agreement with observed properties of liquid water as diverse as density, internal energy, heat capacity, compressibility, pair correlation functions, diffusion coefficient, vibrational spectrum, relative permittivity, and dielectric relaxation spectra. The nonlinear dependence of the relative permittivity on the electric field is also computed by means of fluctuation–dissipation relations.

Computer simulation results for thermodynamic excess properties in fluid mixtures II. Effects of energy parameter differences in simple binary mixtures

This paper reports results from isothermal-isobaric molecular dynamics simulations of binary mixtures of Lennard-Jones fluids. The simulations were performed for size-parameter ratios in the range 1ˇ-σ BB /σ AA ˇ-1·5 and energy-parameter ratios 1ˇ-e BB /e AA ˇ-2. The principal results are excess Gibbs free energies, excess enthalpies, and excess volumes, which are used to test a first-order perturbation theory based on hard sphere mixtures.

Computer simulation results for thermodynamic excess properties in fluid mixtures

This paper reports results from isothermal-isobaric molecular dynamics simulations of binary mixtures of Lennard-Jones fluids. The simulations were performed at several component size ratios in the range 1 ˇ- σ BB /σ AA ˇ- 2 and at three pressures. The principal results reported here are excess Gibbs free energies g E, excess volumes v E, and excess enthalpies h E. These new results do not coincide with previous Lennard-Jones simulations because of the extensive size and pressure ranges studied and because of the accuracy attained in the values for g E. The simulation data are used to test a recent revision of thermodynamic perturbation theory based on hard-sphere mixtures.

Isothermalisobaric molecular dynamics simulation of diatomic liquids and their mixtures

Isothermalisobaric ensemble molecular dynamics simulations have been performed for systems of two-center Lennard-Jones molecules for pure fluids as well as binary mixtures. The results obtained from various ensemble averages have been compared for pure fluid simulations of Lennard-Jones model diatomic fluids. The excess enthalpy and excess volume of three equimolar mixtures (argonnitrogen, argonoxygen, and nitrogenoxygen) have been calculated and compared with values obtained from previous NVT molecular dynamics and perturbation theory. Pair distribution functions obtained from various methods are compared for the equimolar mixture of nitrogen and oxygen and used to study the effect of attractive forces on the local structure. For four other systems (argonethane, methaneethane, carbon disulfidecarbon tetrachloride, and carbon disulfidetetrachloroethylene), excess enthalpies and excess volumes from NPT simulations are used to test the ability of perturbation theory to predict these properties and are also compared with experimental data. The comparison of simulation and experiment clearly shows the need to improve the available parameters for cross interactions in binary mixtures.