Anisotropic United Atom Model Research Articles

Conservative and dissipative force field for simulation of coarse-grained alkane molecules: A bottom-up approach

We apply operational procedures available in the literature to the construction of coarse-grained conservative and friction forces for use in dissipative particle dynamics (DPD) simulations. The full procedure rely on a bottom-up approach: large molecular dynamics trajectories of n-pentane and n-decane modeled with an anisotropic united atom model serve as input for the force field generation. As a consequence, the coarse-grained model is expected to reproduce at least semi-quantitatively structural and dynamical properties of the underlying atomistic model. Two different coarse-graining levels are studied, corresponding to five and ten carbon atoms per DPD bead. The influence of the coarse-graining level on the generated force fields contributions, namely, the conservative and the friction part, is discussed. It is shown that the coarse-grained model of n-pentane correctly reproduces self-diffusion and viscosity coefficients of real n-pentane, while the fully coarse-grained model for n-decane at ambient temperature over-predicts diffusion by a factor of 2. However, when the n-pentane coarse-grained model is used as a building block for larger molecule (e.g., n-decane as a two blobs model), a much better agreement with experimental data is obtained, suggesting that the force field constructed is transferable to large macro-molecular systems.

Chemical Potential of Benzene Fluid from Monte Carlo Simulation with Anisotropic United Atom Model

The profile of chemical potential of benzene fluid has been investigated using Anisotropic United Atom (AUA) model. A Monte Carlo simulation in canonical ensemble was done to obtain the isotherm of benzene fluid, from which the excess part of chemical potential was calculated. A surge of potential energy is observed during the simulation at high temperature which is related to the gas-liquid phase transition. The isotherm profile indicates the tendency of benzene to condensate due to the strong attractive interaction. The results show that the chemical potential of benzene rapidly deviates from its ideal gas counterpart even at low density.

Factors influencing properties of interfacial regions in semicrystalline polyethylene: A molecular dynamics simulation study

Molecular dynamics simulations have been used to prepare semicrystalline samples of linear polyethylene (PE) using the anisotropic united atom (AUA4) model. The initial configurations of the simulations were obtained by controlled melting of the pure crystalline system and a unique re-connection method between chain segments in the intercrystalline regions. Systems containing two crystalline slabs and two large intercrystalline regions are built with different defects, introduced to mimic the structure of real PE systems, namely: bridging chains connecting two different crystalline slabs, re-entrant chains emerging and re-entering into the same crystalline slab, and free chains. We observe large structural differences between the different systems. We show that average and local density depend on defect types which in turn affect the dynamical properties. This explicit simulation of a crystalline–amorphous interfacial region thus gives a possible picture of intercrystalline regions that can serve to understand and predict penetrant gases transport in such materials.

Prediction of the Surface Tension of the Liquid−Vapor Interface of Alcohols from Monte Carlo Simulations

We report the calculation of the surface tension of the liquid−vapor interface of alcohols by two-phase Monte Carlo simulations. The anisotropic united atom model (AUA4) extended to alcohol is applied here for the prediction of the surface tension for alcohols ranging from methanol to octan-1-ol and phenol. The surface tension is calculated using different mechanical and thermodynamic approaches with specific long-range corrections (LRC) used for each definition. Liquid−vapor coexistence curves are also determined from these two-phase simulations and compared with Monte Carlo simulations carried out in the Gibbs ensemble. These calculations represent a genuine test of transferability for this force field since the surface tension is not a property used in the adjustment procedure for the development of the parameters. The surface tension, saturated liquid densities, and critical points compare very well with experiments along the liquid−vapor saturation curve.

Surface tension of water–alcohol mixtures from Monte Carlo simulations

Monte Carlo simulations are reported to predict the dependence of the surface tension of water-alcohol mixtures on the alcohol concentration. Alcohols are modeled using the anisotropic united atom model recently extended to alcohol molecules. The molecular simulations show a good agreement between the experimental and calculated surface tensions for the water-methanol and water-propanol mixtures. This good agreement with experiments is also established through the comparison of the excess surface tensions. A molecular description of the mixture in terms of density profiles and hydrogen bond profiles is used to interpret the decrease of the surface tension with the alcohol concentration and alcohol chain length.

Extension of a Charged Anisotropic United Atoms Model to Polycyclic Aromatic Compounds

A new potential model for polycyclic aromatic hydrocarbons has been developed on the basis of a charged anisotropic united atoms (AUA) potential with six AUA force centers and three electrostatic point charges per aromatic ring. Using quantum mechanical calculations, quadrupolar moments of several aromatic molecules were computed and a correlation has been observed that links the magnitude of the point charges with respect to the number of aromatic rings. The Lennard-Jones parameters of quaternary carbon atoms bridging two aromatic rings have been optimized with the minimization of a dimensionless error criterion incorporating various thermodynamic data of naphthalene. The new potential model, called ch-AUA, was then evaluated on its abilities to predict thermodynamic and transport properties for a series of polycyclic aromatic compounds in a wide range of temperatures. Although the relative errors with respect to the experimental density, vaporization enthalpy, and vapor pressure data are similar to those computed with the noncharged AUA potential, the new ch-AUA potential noticeably improves the prediction of the shear viscosities of polycyclic aromatic compounds. Comparisons between experimental viscosities of 1-methylnaphthalene at different pressures and those computed using the new ch-AUA and the noncharged AUA potentials show that the new potential improves the prediction of viscosities at high pressures.

Calculation of the surface tension of cyclic and aromatic hydrocarbons from Monte Carlo simulations using an anisotropic united atom model (AUA)

We report the calculation of the surface tension of cycloalkanes and aromatics by direct two-phase MC simulations using an anisotropic united atom model (AUA). In the case of aromatics, the polar version of the AUA-4 (AUA 9-sites) model is used. A comparison with the nonpolar models is carried out on the surface tension of benzene. The surface tension is calculated from different routes: the mechanical route using the Irving and Kirkwood (IK) and Kirkwood-Buff (KB) expressions; the thermodynamic route by using the test-area (TA) method. The different operational expressions of these definitions are presented with those of their long range corrections. The AUA potential allows to reproduce very well the dependence of the surface tension with respect to the temperature for cyclopentane, cyclohexane, benzene and toluene.

Surface Tensions of Linear and Branched Alkanes from Monte Carlo Simulations Using the Anisotropic United Atom Model

The anisotropic united atoms (AUA4) model has been used for linear and branched alkanes to predict the surface tension as a function of temperature by Monte Carlo simulations. Simulations are carried out for n-alkanes ( n-C5, n-C6, n-C7, and n-C10) and for two branched C7 isomers (2,3-dimethylpentane and 2,4-dimethylpentane). Different operational expressions of the surface tension using both the thermodynamic and the mechanical definitions have been applied. The simulated surface tensions with the AUA4 model are found to be consistent within both definitions and in good agreement with experiments.

Optimized intermolecular potential for nitriles based on Anisotropic United Atoms model

An extension of the anisotropic united atoms intermolecular potential model is proposed for nitriles. The electrostatic part of the intermolecular potential is calculated using atomic charges obtained by a simple Mulliken population analysis. The repulsion-dispersion interaction parameters for methyl and methylene groups are taken from transferable AUA4 literature parameters [Ungerer et al., J. Chem. Phys., 2000, 112, 5499]. Non-bonding Lennard-Jones intermolecular potential parameters are regressed for the carbon and nitrogen atoms of the nitrile group (-C[triple bound] N) from experimental vapor-liquid equilibrium data of acetonitrile. Gibbs Ensemble Monte Carlo simulations and experimental data agreement is very good for acetonitrile, and better than previous molecular potential proposed by Hloucha et al. [J. Chem. Phys., 2000, 113, 5401]. The transferability of the resulting potential is then successfully tested, without any further readjustment, to predict vapor-liquid phase equilibrium of propionitrile and n-butyronitrile.

Optimisation of the dynamical behaviour of the anisotropic united atom model of branched alkanes: application to the molecular simulation of fuel gasoline

In the present work, we have optimised the dynamical behaviour of the anisotropic united atom (AUA) intermolecular potential for branched alkanes developed by Bourasseau et al. [E. Bourasseau, P. Ungerer, A. Boutin, and A.H. Fuchs, Monte Carlo simulation of branched alkanes and long chain n-alkanes with anisotropic united atoms intermolecular potential, Mol. Sim. 28 (2002), pp. 317–336], by a modification of the energetic barrier of the torsion potential. The new 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 reproduces transport properties like the shear viscosity, keeping the accuracy achieved in the original work for equilibrium properties. An extensive investigation of the shear viscosity of 12 different types of branched alkanes in a wide range of pressures and temperatures, shows that the AUA(4m) improves the accuracy of the original AUA4, reducing the absolute average deviation from 24 to 15%. In addition, molecular simulation results of the shear viscosity of olefins reveal that the original AUA potential is accurate enough to reproduce the experimental data with less than 12% of deviation. Finally, we present a consistent lumping methodology to perform molecular simulations on complex multi-component systems such as fuel gasoline by representing the real system by a simplified mixture with only tenths of species.

Anisotropic United Atom Model Including the Electrostatic Interactions of Methylbenzenes. II. Transport Properties

A new potential model for monoaromatics based an anisotropic united atoms model and three electrostatic charges is used to simultaneously represent the dipole moments, the vapor−liquid equilibrium, the transport properties, and the structural properties while keeping computational effort minimal. The present article focuses on the transport properties and dynamic structural properties. Shear viscosities and diffusion coefficients are computed from equilibrium molecular dynamics in a large range of pressures (0.1−500 MPa) and temperatures (298−363 K). Viscosity shows a significant improvement with respect to the previous nonpolar models, as average deviations on toluene, p-xylene, m-xylene, and o-xylene stay below 9%. Self-diffusion coefficients and reorientational diffusion coefficients of toluene are also in good agreement with experimental observations. Thermal conductivity, evaluated by nonequilibrium molecular dynamics, reveals maximum deviations of 8% on benzene, toluene, and xylene isomers, which is better than those of other potentials investigated. By its lower computation time and equivalent accuracy compared with all atoms models, the proposed AUA potential is a good option for investigating complex problems such as the prediction of fuel viscosity in the high-pressure conditions encountered in the injectors of diesel engines.

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.

Anisotropic diffusion of n-butane and n-decane on a stepped metal surface

The diffusion of single n-butane and n-decane molecules on a model stepped surface, Pt655, and on a corresponding flat surface, Pt111, is investigated using molecular-dynamics simulations and anisotropic united atom model. The surface step on Pt655 causes the alkane molecules to adsorb on the lower terrace in all-trans conformations with their long molecular axes adjacent and parallel to the step edge, and to diffuse anisotropically along the surface step via a constant wiggly motion without rotation or marked deviation from the parallel adsorption configuration. At relatively high temperatures, the alkane molecules can temporarily break away from the step edge but cannot migrate across the step edge in either the downstair or upstair direction. In comparison with the diffusion on Pt111, the diffusivity of n-decane is reduced by the surface step but its diffusion barrier is hardly affected. In the case of the shorter n-butane, however, the surface step significantly reduces the diffusion energy barrier and gives rise to higher diffusion coefficients at lower temperatures. Important implications of the simulation results are discussed.

Dynamical and structural properties of benzene in supercritical water

We have employed an anisotropic united atom model of benzene (R. O. Contreras, Ph.D. thesis, Universitat Rovira i Virgili 2002) that reproduces the quadrupolar moment of this molecule through the inclusion of seven point charges. We show that this kind of interaction is required to reproduce the solvation of these molecules in supercritical water. We have computed self-diffusion coefficient and Maxwell-Stefan coefficients as well as the shear viscosity for the mixture water-benzene at supercritical conditions. A strong density and composition dependence of these properties is observed. In addition, our simulations are in qualitative agreement with the experimental evidence that, at medium densities (0.6 g/cm(3) and 673 K), almost half of the benzene molecules have one hydrogen bond with water molecules. We also observe that these bonds are longer lived than the corresponding hydrogen bonds between water molecules. Similarly, we obtain an important reduction of the dielectric constant of the mixture with the increment of the amount of benzene molecules at medium and high densities.

Optimized Intermolecular Potential for Aromatic Hydrocarbons Based on Anisotropic United Atoms. 2. Alkylbenzenes and Styrene

An optimization has been performed for the parameters of an Anisotropic United Atoms (AUA) intermolecular potential for thermodynamic property prediction using both Gibbs ensemble and NPT Monte Carlo simulations. The model uses the same parameters as previous AUA models for the aromatic CH, alkyl CH2, and methyl CH3 groups as well as the CH and CH2 groups for olefins. The optimization procedure is based on the minimization of a dimensionless error criterion incorporating various thermodynamic data of p-xylene at 311 and 491 K. The model has been evaluated on a series of alkylbenzenes and styrene including toluene, o- and m-xylene, trimethylbenzene, n-propylbenzene, n-hexylbenzene, and n-dodecylbenzene from ambient temperature to near-critical temperature. Vaporization enthalpy, liquid density, and normal boiling temperature are reproduced with an average error of 2% or lower. Although the proposed AUA model is very simple, as it does not include any electrostatic charges, it accounts fairly well for the influence of the alkyl substituents over a large range of temperature and carbon number.

AUA model NEMD and EMD simulations of the shear viscosity of alkane and alcohol systems

The viscosities of alkanes (propane, isobutane, nonane), alcohols (ethanol, propanol, isopropanol, 2-butanol) and isopropanol+nonane mixtures were calculated using non-equilibrium and equilibrium molecular dynamics (NEMD and EMD) simulation methods. The nonane, isopropanol and nonane+isopropanol mixture simulations were performed in response to the First Industrial Simulation Challenge. Intermolecular interactions were modeled using an anisotropic united-atom Buckingham exponential-6 potential. The force field parameters were optimized using pure component viscosity data. The resulting viscosities are compared with literature values, with the result that the anisotropic united-atom model together with the exponential-6 model can give good predictions of the viscosity of n-alkane and alcohol systems.

Atomistic Monte Carlo simulation of strictly monodisperse long polyethylene melts through a generalized chain bridging algorithm

This work is concerned with the atomistic simulation of the volumetric, conformational and structural properties of monodisperse polyethylene (PE) melts of molecular length ranging from C78 up to C1000. In the past, polydisperse models of these melts have been simulated in atomistic detail with the end-bridging Monte Carlo algorithm [Pant and Theodorou, Macromolecules 28, 7224 (1995); Mavrantzas et al., Macromolecules 32, 5072 (1999)]. In the present work, strictly monodisperse as well as polydisperse PE melts are simulated using the recently introduced double bridging and intramolecular double rebridging chain connectivity-altering Monte Carlo moves [Karayiannis et al., Phys. Rev. Lett. 88, 105503 (2002)]. These algorithms constitute generalizations of the EB move, since they entail the construction of two trimer bridges between two properly chosen pairs of dimers along the backbones of two different chains or along the same chain. In the simulations, a new molecular model is employed which is a hybrid of the united-atom TraPPE model [Martin and Siepmann, J. Phys. Chem. B 102, 2569 (1998)] and the anisotropic united-atom model [Toxvaerd, J. Chem. Phys. 107, 5197 (1997)]. Results are first presented documenting the efficiency of the algorithm in equilibrating long-chain PE melts and its dependence on chain length and polydispersity. Simulation data concerning the volumetric, conformational and structural properties of the monodisperse PE melts, obtained with the new simulation algorithm, are found to be in excellent agreement with available experimental data.

Vapour-Liquid Phase Equilibria of n-alkanes by Direct Monte Carlo Simulations

We report results of direct Monte Carlo simulations of n-pentane and n-decane at the liquidvapour interface for a number of temperatures. The intermolecular interactions are modeled using the last version of the anisotropic united atom model (AUA4). We have used the local long range correction energy and an algorithm allowing to select randomly with equal probability two different displacements. The liquid and vapour densities are in excellent agreement with experimental data and with those previously calculated using the GEMC method.

Derivation of an Optimized Potential Model for Phase Equilibria (OPPE) for Sulfides and Thiols

An extension of the anisotropic united atoms model is proposed for thiols and sulfides. A complete derivation of the nonbonded parameters is performed with the aim of obtaining a transferable force field. The electrostatic part of the intermolecular potential is represented by a set of atomic charges. These charges are extracted from quantum chemical calculations thanks to a method developed recently. These charges are shown to depend very weakly on the conformation of the molecule. The repulsion−dispersion interactions are described by an anisotropic united atom Lennard-Jones potential: parameters for methyl and methylene groups are directly taken from a previous study on alkanes, whereas parameters for −S and −SH groups are fitted to experimental data. The resulting potential, which we will term OPPE (optimized potential model for phase equilibria), is tested against liquid properties at atmospheric pressure and vapor−liquid-phase equilibria of various sulfides and thiols to prove its transferability. Excellent agreement is obtained with experimental data.

Optimization of the anisotropic united atoms intermolecular potential for n-alkanes

The parameters of the anisotropic united atoms potential for linear alkanes proposed by Toxvaerd [S. Toxvaerd, J. Chem. Phys. 107, 5197 (1997)] have been optimized on the basis of selected equilibrium properties (vapor pressures, vaporization enthalpies, and liquid densities) of ethane, n-pentane, and n-dodecane. The optimized parameters for the CH2 and CH3 groups form a regular sequence with those of methane and the force centers are found between the carbon and hydrogen atoms, as expected. The resulting potential, called AUA4, has been compared with Toxvaerd’s potential (AUA3) by using several molecular simulation methods (Gibbs ensemble Monte Carlo, thermodynamic integration, and molecular dynamics). An investigation performed at temperatures ranging from 140 to 700 K and with various chain lengths up to 20 carbon atoms has shown AUA4 to provide systematic improvements of vapor pressures, vaporization enthalpies, and liquid densities for pure n-alkanes. Significant improvements have been also noticed on the critical temperatures of n-alkanes, estimated from coexistence density curves, and on the equilibrium properties of CO2–n-alkane binary mixtures. Self-diffusion coefficients of n-hexane, however, are slightly improved by the new potential, but still exceed experimental measurements at low temperature. As we have only optimized the intermolecular potential in the present study, it is suggested that further optimization of the intramolecular potentials of the anisotropic united atoms model could allow simultaneous prediction of thermodynamic properties and of transport coefficients, particularly in very dense liquids.