Richard J. Sadus

Molecular simulation of the phase behavior of noble gases using accurate two-body and three-body intermolecular potentials

Gibbs ensemble Monte Carlo simulations are reported for the vapor–liquid phase coexistence of argon, krypton, and xenon. The calculations employ accurate two-body potentials in addition to contributions from three-body dispersion interactions resulting from third-order triple-dipole, dipole–dipole–quadrupole, dipole–quadrupole–quadrupole, quadrupole–quadrupole–quadrupole, and fourth-order triple-dipole terms. It is shown that vapor–liquid equilibria are affected substantially by three-body interactions. The addition of three-body interactions results in good overall agreement of theory with experimental data. In particular, the subcritical liquid-phase densities are predicted accurately.

Solid-liquid equilibria and triple points of n-6 Lennard-Jones fluids

Molecular dynamics simulations are reported for the solid-liquid coexistence properties of n-6 Lennard-Jones fluids, where n=12, 11, 10, 9, 8, and 7. The complete phase behavior for these systems has been obtained by combining these data with vapor-liquid simulations. The influence of n on the solid-liquid coexistence region is compared using relative density difference and miscibility gap calculations. Analytical expressions for the coexistence pressure, liquid, and solid densities as a function of temperature have been determined, which accurately reproduce the molecular simulation data. The triple point temperature, pressure, and liquid and solid densities are estimated. The triple point temperature and pressure scale with respect to 1/n, resulting in simple linear relationships that can be used to determine the pressure and temperature for the limiting infinity-6 Lennard-Jones potential. The simulation data are used to obtain parameters for the Raveche, Mountain, and Streett and Lindemann melting rules, which indicate that they are obeyed by the n-6 Lennard Jones potentials. In contrast, it is demonstrated that the Hansen-Verlet freezing rule is not valid for n-6 Lennard-Jones potentials.

Viscoelastic properties of dendrimers in the melt from nonequlibrium molecular dynamics

The viscoelastic properties of dendrimers of generation 1-4 are studied using nonequilibrium molecular dynamics. Flow properties of dendrimer melts under shear are compared to systems composed of linear chain polymers of the same molecular weight, and the influence of molecular architecture is discussed. Rheological material properties, such as the shear viscosity and normal stress coefficients, are calculated and compared for both systems. We also calculate and compare the microscopic properties of both linear chain and dendrimer molecules, such as their molecular alignment, order parameters and rotational velocities. We find that the highly symmetric shape of dendrimers and their highly constrained geometry allows for substantial differences in their material properties compared to traditional linear polymers of equivalent molecular weight.

Molecular dynamics simulation of the dielectric constant of water: The effect of bond flexibility

The role of bond flexibility on the dielectric constant of water is investigated via molecular dynamics simulations using a flexible intermolecular potential SPC/Fw [Y. Wu, H. L. Tepper, and G. A. Voth, J. Chem. Phys. 128, 024503 (2006)]. Dielectric constants and densities are reported for the liquid phase at temperatures of 298.15 K and 473.15 K and the supercritical phase at 673.15 K for pressures between 0.1 MPa and 200 MPa. Comparison with both experimental data and other rigid bond intermolecular potentials indicates that introducing bond flexibility significantly improves the prediction of both dielectric constants and pressure-temperature-density behavior. In some cases, the predicted densities and dielectric constants almost exactly coincide with experimental data. The results are analyzed in terms of dipole moments, quadrupole moments, and equilibrium bond angles and lengths. It appears that bond flexibility allows the molecular dipole and quadrupole moment to change with the thermodynamic state point, and thereby mimic the change of the intermolecular interactions in response to the local environment.

Internal structure of dendrimers in the melt under shear: A molecular dynamics study

The molecular structure of fluids composed of dendrimers of different generations is studied using nonequilibrium molecular dynamics (NEMD). NEMD results for dendrimer melts undergoing planar Couette flow are reported and analyzed with particular attention paid to the shear-induced changes in the internal structure of dendrimers. The radii of gyration, pair distribution functions and the fractal dimensionality of the dendrimers are determined at different strain rates. The location of the terminal groups is analyzed and found to be uniformly distributed throughout the space occupied by the molecules. The fractal dimension as a function of strain rate displays crossover behavior analogous to the Newtonian/non-Newtonian transition of shear viscosity.

Molecular simulation of the vapor–liquid coexistence of mercury

The vapor‐liquid coexistence properties of mercury are determined from molecular simulation using empirical intermolecular potentials, ab initio two-body potentials, and an effective multibody intermolecular potential. Comparison with experiment shows that pair-interactions alone are inadequate to account for the vapor‐liquid coexistence properties of mercury. It is shown that very good agreement between theory and experiment can be obtained by combining an accurate two-body ab initio potential with the addition of an empirically determined multibody contribution. As a consequence of this multibody contribution, we can reliably predict mercury’s phase coexistence properties and the heats of vaporization. The pair distribution function of mercury can also be predicted with reasonable accuracy.

Influence of bond flexibility on the vapor-liquid phase equilibria of water

The authors performed Gibbs ensemble simulations on the vapor-liquid equilibrium of water to investigate the influence of incorporating intramolecular degrees of freedom in the simple point charge (SPC) water model. Results for vapor pressures, saturation densities, heats of vaporization, and the critical point for two different flexible models are compared with data for the corresponding rigid SPC and SPC/E models. They found that the introduction of internal vibrations, and also their parametrization, has an observable effect on the prediction of the vapor-liquid coexistence curve. The flexible SPC/Fw model, although optimized to describe bulk diffusion and dielectric constants at ambient conditions, gives the best prediction of saturation densities and the critical point of the examined models.

Molecular dynamics simulation of the effect of bond flexibility on the transport properties of water

Molecular dynamics simulations for the shear viscosity and self-diffusion coefficient of pure water were performed to investigate the effect of including intramolecular degrees of freedom in simple point charge (SPC) models over a wide range of state points. Results are reported for the flexible SPC/Fw model, its rigid SPC counterpart, and the widely used SPC/E model. The simulations covered the liquid phase from 277.15 to 363.15 K and the supercritical phase at 673.15 K and pressures up to 200 MPa. The flexibility exhibited by the SPC/Fw model results in slowing down of the dynamics. That is, it results in higher shear viscosities and lower diffusion coefficients than can be obtained from the rigid model, resulting in better agreement with experimental data. Significantly, the SPC/Fw model can be used to adequately predict the diffusion coefficients at ambient and supercritical temperatures over a wide range of pressures.

Exact calculation of the effect of three-body Axilrod–Teller interactions on vapour–liquid phase coexistence

Abstract The Gibbs ensemble algorithm is implemented to determine the vapour–liquid phase coexistence of a pure fluid interacting via a two-body Lennard–Jones+three-body Axilrod–Teller potential. The contribution of both two-body and three-body interactions are calculated exactly. The results are compared with both experiment and two-body only simulation data. The position of the vapour branch of the coexistence curve is almost unaffected by the inclusion of three-body interactions. In contrast, the liquid branch occurs at substantially lower densities compared with Lennard–Jones simulation data. However, the approach to the critical point is improved by including three-body interactions, and the estimated critical point is in good agreement with the experiment.

Three‐body interactions in fluids from molecular simulation: Vapor–liquid phase coexistence of argon

Gibbs‐ensemble molecular simulations are reported for the vapor–liquid phase coexistence of argon using the two‐body Lennard‐Jones potential. During the simulation, the possible effect of three‐body interactions on the pressure and configurational energy of the vapor and liquid phases is estimated by performing calculations with three‐body potentials. The intermolecular potentials used for the three‐body calculations incorporate the influence of both three‐body‐dispersion and three‐body‐repulsion interactions. The results show that three‐body repulsion makes a significant contribution to three‐body interactions in the liquid phase. The effect of three‐body dispersion is offset substantially by three‐body repulsion.