Showing papers by "Peter T. Cummings published in 2002"

Comparison of Nonequilibrium Molecular Dynamics with Experimental Measurements in the Nonlinear Shear-Thinning Regime

Abstract: Nonequilibrium molecular dynamics of a low-molecular-weight fluid (squalane) are compared with experimental measurements in both the linear (Newtonian) and nonlinear (non-Newtonian) regimes. The experimental and simulation data are shown to follow the same time-temperature superposition master curve. This represents the first comparison of the nonlinear rheology predicted by nonequilibrium molecular dynamics with experiment, and is thus the first experimental test of nonequilibrium molecular dynamics simulations in the nonlinear regime.

Molecular dynamics study of the structure and thermophysical properties of model sI clathrate hydrates

Abstract: We performed isothermal−isobaric molecular dynamics simulations to study the effects of host−guest asymmetries on the thermophysical and structural properties of methane and carbon dioxide sI clathrate hydrates. In particular, we analyzed the effect of the strength of the host−guest interactions in realistic molecular descriptions of the host intermolecular potentials, the effect of the type of water intermolecular potential, and the degree of cage occupancy on the resulting properties of the hydrates. Finally, we used the simulation results to interpret the limitations and implications of the main assumptions behind the original vdWP theory of clathrates and its current modifications, which are used in the modeling of hydrate phase equilibria.

Determination of the Gibbs Free Energy of Gas Replacement in SI Clathrate Hydrates by Molecular Simulation

Abstract: The thermodynamic feasibility of extracting CH4 gas from its hydrate clathrate by CO2 replacement is analyzed by molecular dynamics simulation. The approach to this investigation is the proof-of-principles and the development of the molecular tools to study the replacement equilibrium process. The effect of the water (model) description on the free energy of the replacement process is discussed and some relevant implications regarding the real process are addressed.

Calculation of Viscous EHL Traction for Squalane Using Molecular Simulation and Rheometry

Abstract: Recently, remarkable agreement was reported between nonequilibrium molecular dynamics simulation and high-pressure Couette rheometry on squalane. We have utilized the viscosity-strain rate relationship obtained from this unique combination of experimental and simulation data along with high-pressure viscometer measurements to calculate the viscous traction curve in the elastohydrodynamic lubrication (EHL) regime. A comparison with measured traction at 0.57 and 1.29 GPa shows excellent agreement, confirming the validity of the measurements and simulations. Thus, we present for the first time, a successful calculation of EHL traction from the liquid shear response obtained from both molecular dynamics and rheometry.

Effect of the range of interactions on the properties of fluids. Phase equilibria in pure carbon dioxide, acetone, methanol, and water

Abstract: The effect of the long-range Coulombic interactions on the vapor−liquid equilibria properties of polar and associating fluids has been investigated, by considering typical representatives of these classes of fluids, namely, carbon dioxide, acetone, methanol, and water, defined by realistic intermolecular pair potential models. Using the same decomposition of realistic potential models into a short-range part and a residual part as in previous papers [Kolafa, J.; Nezbeda, I. Mol. Phys. 2000, 98, 1505−1520. Kolafa, J.; Nezbeda, I.; Lisal, M. Mol. Phys. 2001, 99, 1751−1764], we carried out Gibbs ensemble simulations on both the full and short-range models to determine the thermodynamic properties of the considered compounds along the vapor−liquid coexistence curve. In addition, we also considered methanol in two homogeneous phases, liquid and supercritical, to determine its structure and thermodynamic properties. We have found that the long-range interactions affect all considered properties only marginally ...

Layering Behavior and Axial Phase Equilibria of Pure Water and Water + Carbon Dioxide Inside Single Wall Carbon Nanotubes

Abstract: We have studied the phase equilibrium and layering behavior of the liquid phase in water and water + CO2 systems inside single wall carbon nanotubes. The system containing pure water was examined at temperatures between 274 and 500 K. At low temperatures and close to the wall of the nanotube, the formation of layers enclosing the liquid phase was observed. Upon addition of CO2, gas-liquid equilibrium was observed, resulting in an enhanced layering effect in the liquid phase.

Predicting the Newtonian viscosity of complex fluids from high strain rate molecular simulations

Abstract: The prediction of viscosity by molecular simulation has been a goal of molecular modeling essentially since its inception. With today’s computing power, the Newtonian or zero shear viscosity of a low molecular weight fluid can easily be determined using equilibrium and nonequilibrium molecular dynamics simulation methods. However, both methods are constrained to systems with relatively short relaxation times that are accessible on the timescale of a molecular dynamics simulation. Here we demonstrate that using a simple scaling relation enables us to predict the Newtonian viscosity of a molecule at any state point for a small fraction of the time that it takes to obtain the same result through nonequilibrium or equilibrium molecular dynamics simulation.

Shear viscosity of a simple fluid over a wide range of strain rates

Abstract: The shear viscosity of the Weeks—Chandler—Andersen (WCA) fluid at the Lennard-Jones triple point has been calculated over a wide range of strain rates using the transient time correlation function (TTCF) formalism. It has been demonstrated that these calculations can be carried out at arbitrarily low strain rates with the precision of the Green—Kubo calculations. At high strain rates, the calculated data agree within the error bars with more precise data acquired using the computationally less demanding steady state (SS) non-equilibrium molecular dynamics (NEMD) method. The linear variation of viscosity with the square root of the strain rate is discussed.

Hydrophobic hydration at the level of primitive models. II: Large solutes and water restructuring

Abstract: Details of structural changes that take place in water near an apolar solute have been studied by Monte Carlo simulations for hard sphere solutes of increasing size, including the limiting case of water at a hard structureless wall. Water has been modelled by two diA erent types of extended primitive model, the four-site EPM4 model and ®ve-site EPM5 model. Two diA erent patterns of the orientational ordering of the water molecules around the solute as a function of its size have been found. For the EPM5 model, the structure of water and the orientation of its molecules near an apolar solute of ®nite diameter do not seem to be sensitive to the size of the solute, and only become more pronounced when the solute becomes a hard wall. On the other hand, the orientation ordering of the EPM4 molecules gradually changes with increasing size of the solute, and for solutes larger than approximately ®ve times the size of the water molecule it is opposite to that near a small solute. A novel method to evaluate the excess chemical potential of large solutes has been implemented, and some thermodynamic quantities for water (distribution of hydrogen bonds and the excess chemical potential) have been computed as a function of the distance from the solute.

Development of a force field for molecular simulation of the phase equilibria of perfluoromethylpropyl ether

Abstract: A first step towards the development of a general, realistic potential model for perfluoroether compounds has been to parameterize a united atom model for a short chain perfluoroether perfluoromethylpropyl ether (CF3CF2CF2OCF3). The potential model takes the usual form in which separate bond bending and torsional terms describe the intramolecular interactions with the addition of van der Waals and electrostatic terms to describe the non-bonded interactions. Ab initio quantum calculations have been carried out to obtain the partial charges and intramolecular torsional and bending potentials. Phase equilibrium data were then used to optimize the van der Waals interaction parameters through Gibbs ensemble Monte Carlo simulations. The resulting model reproduces vapour-liquid equilibrium densities, the critical temperature and the critical density of perfluoromethylpropyl ether, in good agreement with those from experiment.

Pair approximation for polarization interaction and adiabatic nuclear and electronic sampling method for fluids with dipole polarizability

Abstract: The adiabatic nuclear and electronic sampling method (ANES), originally formulated as an efficient Monte Carlo algorithm for systems with fluctuating charges, is applied to the simulation of a polarizable water model with induced dipole moments. Structural, thermodynamic and dipolar properties obtained by ANES and a newer algorithm, the pair approximation for polarization interaction (PAPI), are compared with full iteration. With the best parameters, the inaccuracy of both approximate methods was found to be comparable with the uncertainty of the full iteration. The PAPI method with iteration radius equal to the second minimum of the oxygen—oxygen correlation function is, depending on the convergence tolerance, 10–15 times faster than the full iteration for 256 molecules, and yields very accurate structure and thermodynamics with deviation about 0.3%. When the iteration radius is increased to the cutoff distance, exact results are recovered at the cost of decreased efficiency. The ANES method with small n...

Applications of Molecular and Materials Modeling

Abstract: : This report reviews the development and applications of molecular and materials modeling in Europe and Japan in comparison to those in the United States. Topics covered include computational quantum chemistry, molecular simulations by molecular dynamics and Monte Carlo methods, mesoscale modeling of material domains, molecular-structure/macroscale property correlations like QSARs and QSPRs, and related information technologies like informatics and special-purpose molecular-modeling computers. The panel's findings include the following: The United States leads this field in many scientific areas. However, Canada has particular strengths in DFT methods and homogeneous catalysis; Europe in heterogeneous catalysis, mesoscale, and materials modeling; and Japan in materials modeling and special-purpose computing. Major government-industry initiatives are underway in Europe and Japan, notably in multi-scale materials modeling and in development of chemistry-capable ab-initio molecular dynamics codes. In European and U.S. assessments of nanotechnology, it was also concluded that to advance the field most quickly and competitively the need is acute for applying new and existing methods of molecularly based modeling. Additional findings are outlined in the panel's executive summary.

Structural and thermodynamic properties of a multicomponent freely jointed hard sphere multi-Yukawa chain fluid

Abstract: The product±reactant Ornstein±Zernike approach, represented by the polymer mean-spherical approximation (PMSA), is utilized to describe the structure and thermodynamic properties of theuid of Yukawa hard sphere chain molecules. An analytical solution of the PMSA for the most general case of the multicomponent freely jointed hard sphere multi-Yukawa chainuid is presented. As in the case of the regular MSA for the hard sphere Yukawauid, the problem is reduced to the solution of a set of nonlinear algebraic equations in the general case, and to a single equation in the case of the factorizable Yukawa potential coeA cients. Closed form analytical expressions are presented for the contact values of the monomer±monomer radial distribution function, structure factors, internal energy, Helmholtz free energy, chemical potentials and pressure in terms of the quantities, which follows directly from the PMSA solution. By way of illustration, several diA erent versions of the hard sphere Yukawa chain model are considered, represented by one-Yukawa chains of length m, where m ˆ 2;4;8;16. To validate the accuracy of the present theory, Monte Carlo simulations were carried out and the results are compared systematically with the theoretical results for the structure and thermodynamic properties of the system at hand. In general it is found that the theory per- forms very well, thus providing an analytical route to the equilibrium properties of a well de®ned model for chainuids.

Theory and modeling in nanoscience: Report of the May 10-11, 2002 Workshop

Abstract: On May 10 and 11, 2002, a workshop entitled ''Theory and Modeling in Nanoscience'' was held in San Francisco, California, sponsored by the offices of Basic Energy Science and Advanced Scientific Computing Research of the Department of Energy. The Basic Energy Sciences Advisory Committee and the Advanced Scientific Computing Advisory Committee convened the workshop to identify challenges and opportunities for theory, modeling, and simulation in nanoscience and nanotechnology, and additionally to investigate the growing and promising role of applied mathematics and computer science in meeting those challenges. This report is the result of those contributions and the discussions at the workshop.

Keith E. Gubbins: A celebration of statistical mechanics

Abstract: On the occasion of Keith Gubbins’ 65th birthday, we are delighted and honoured to have the opportunity to introduce these two issues of Molecular Physics to celebrate his achievements and contributions over the last four decades. Keith is a renowned figure in the area of the statistical mechanics of fluids, with key papers that have established lines of investigation for countless researchers. But Keith is known also for his support of younger researchers in the field, his dedication to international collaboration, and his contributions in the development of the institutions and departments of which he has been part. The papers by his collaborators, friends and colleagues collected in these three Special Issues of Molecular Physics offer but a small glimpse of the high regard in which Keith is held. Keith grew up in Southampton, then began his academic training at Queen Mary College, University of London, where he received his B.Sc. in Chemistry with First Class Honours in 1958. This was followed by his Ph.D. in Chemical Engineering from King’s College, University of London, in 1962. The topic of his postgraduate research, which was largely experimental, was the kinetics of reactions in fluidized beds. He then went to the University of Florida as a postdoctoral fellow to work on the mass transport of gases in fuel cells (in particular, transport through the porous electrodes and through the electrolyte) with Robert Walker. He became a faculty member at Florida in 1964, rapidly rising through the ranks to become a full professor in 1972. When he joined the faculty at Florida, he was asked to teach thermodynamics, and this was how he became interested in the statistical mechanics of fluids. Much of his early interest in statistical mechanics was focused on the development of a theoretical framework for the diffusion and solubility of gases in electrolyte solutions. While at the University of Florida, with the late Tim Reed he co-wrote the book Applied Statistical Mechanics (McGraw-Hill, 1973). This was a popular graduate level text among the emerging (but at that time small) community of researchers in statistical mechanics in the field of chemical engineering. Also while at Florida, he began a long and very productive collaboration with Chris Gray of the University of Guelph. In 1976 Keith moved to Cornell University as T. R. Briggs Professor of Chemical Engineering, where he stayed for 22 years, serving 7 years as department head (1983-1990). While at Cornell, Keith established a style of conducting research that has become synonymous with him. He made excellent use of Cornell’s resources and those of various funding agencies to build a vibrant international collaborative laboratory in statistical mechanics and molecular modelling, a tradition he is continuing at North Carolina State University, where he became W. H. Clark Distinguished University Professor in 1998. It is not unusual for Keith to have 5 or 6 international visitors a year, interacting with him over a wide range of areas. These visitors often become involved in his research grants and the research of his students and postdoctoral researchers. For example, one of us (J.S.R.) was appointed the Mary Upson Professor of Engineering in Keith’s department in 1988, and came back with a wider appointment as Andrew D. White Professor-atLarge from 1990 to 1996. As a result he visited Cornell many times from 1970 onwards. Another of us (G.J.) was a visitor to Cornell in 1986 before joining Keith’s group as a post-doctoral researcher for two years beginning in 1987. In the early 1970s, Keith’s research interests gradually retreated from transport properties of gases in solution and turned to the development of a perturbation theory for molecular fluids. In much of the latter work he collaborated closely with Chris Gray of Guelph, and

RESEARCH NOTE Shear viscosity of a simpleuid over a wide range of strain rates

Abstract: The shear viscosity of the Weeks±Chandler±Andersen (WCA) ¯uid at the Lennard-Jones triple point has been calculated over a wide range of strain rates using the transient time correlation function (TTCF) formalism. It has been demonstrated that these calculations can be carried out at arbitrarily low strain rates with the precision of the Green±Kubo calculations. At high strain rates, the calculated data agree within the error bars with more precise data acquired using the computationally less demanding steady state (SS) non-equilibrium molecular dynamics (NEMD) method. The linear variation of viscosity with the square root of the strain rate is discussed.

Characterizing the high pressure behaviour of lubricant basestocks

Abstract: Department of Chemical Engineering, Colorado School of Mines, Golden CO 80401 Departments of Chemical Engineering, Chemistry and Computer Science, University of Tennessee, Knoxville TN 37996 Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge TN George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta GA 30332 Department of Chemical Engineering and Materials Science, Wayne State University, Detroit MI 48202