Peter J. Daivis

Comparison of constant pressure and constant volume nonequilibrium simulations of sheared model decane

We present the results of nonequilibrium molecular dynamics simulations of a model decane fluid performed at constant pressure and compare them with results previously obtained from simulations performed at constant volume. The strain rate dependence of the viscosity at constant pressure is found to differ from that obtained previously at constant volume. The shear thickening at high strain rates observed in constant volume simulations vanishes when the simulations are performed at constant pressure. We also investigate the question of how our low strain rate data for decane can be accurately extrapolated to zero strain rate. We find a well defined first Newtonian region in which the viscosity is independent of strain rate to within errors. The value of the viscosity that we obtain in this region agrees well with the zero strain rate viscosity calculated from the Green–Kubo formula at equilibrium.

THERMOSTATS FOR MOLECULAR FLUIDS UNDERGOING SHEAR FLOW: APPLICATION TO LIQUID CHLORINE

In this article we show that atomic thermostats which have been used in the past for nonequilibrium molecular dynamics (NEMD) simulations of molecular fluids were incorrectly formulated. The error stems from an incorrect assumption made about the form of the streaming angular velocity. This assumption is incorrect even in the linear regime. One spurious effect of this atomic thermostat is the generation of a nonsymmetric pressure tensor. We outline a general method, based on a variational principle, for calculating the position and orientation dependent streaming velocity. Using this streaming velocity we develop an atomic thermostat for molecular fluids which does not bias the positional or orientational distribution functions for the fluid. The new atomic thermostat is validated in NEMD simulations of liquid chlorine undergoing planar Couette flow.

Computer simulation algorithms for molecules undergoing planar Couette flow: A nonequilibrium molecular dynamics study

Results from extensive nonequilibrium molecular dynamics (NEMD) simulations are presented for liquid chlorine subject to planar Couette flow. Comparisons are made between the so‐called atomic and molecular SLLOD algorithms [R. Edberg, G. P. Morriss, and D. J. Evans, J. Chem. Phys. 86, 4555 (1987)] with atomic and molecular thermostats, respectively. These two thermostats differ in the assumptions that are made regarding the streaming velocity. Both thermostats are responsible for the production of string phases characterized by a translational ordering at very high strain rates. In addition, the atomic thermostat is responsible for the existence of a nonvanishing antisymmetric stress and enhanced orientational ordering.

A STUDY OF VISCOSITY INHOMOGENEITY IN POROUS MEDIA

The theory of transport in highly inhomogeneous systems, developed recently by Pozhar and Gubbins, and the nonequilibrium molecular dynamics (NEMD) technique are employed to study the viscosity of WCA fluids confined in narrow slit pores of width 5.1 and 20σ at reduced densities ρσ3 of 0.422–0.713. Calculated quantities include the equilibrium and nonequilibrium density profiles, equilibrium pair correlation functions, flow velocity profiles, and the viscosity profiles. NEMD simulation results are compared with the theoretical predictions. The agreement is good except for the region within one molecular diameter from the walls. The viscosity was found to vary with position across the pore.

Non-equilibrium molecular dynamics calculation of thermal conductivity of flexible molecules: butane

We introduce a new version of the Evans non-equilibrium molecular dynamics algorithm for thermal conductivity that can be applied to molecules with arbitrary internal degrees of freedom including rigid-body rotation. The algorithm is tested by calculating the thermal conductivity of a model butane fluid. It gives a value in good agreement with that obtained from the Green-Kubo formula evaluated at equilibrium and also with the experimental value.

Transport coefficients of liquid butane near the boiling point by equilibrium molecular dynamics

We present very precise results for the linear viscosity, thermal conductivity, and self‐diffusion coefficient of the Ryckaert–Bellemans model of liquid butane near the boiling point, calculated from their respective Green–Kubo formulas using equilibrium molecular dynamics simulations. These results are used as a basis for the appraisal of previous calculations of these transport coefficients, which vary considerably. We find excellent agreement between our results and the results of the most precise nonequilibrium molecular dynamics simulations. We directly examine the system‐size dependence of these transport coefficients for system sizes between 64 and 864 molecules and find that it is negligible, within experimental errors, for the viscosity and the thermal conductivity. The self‐diffusion coefficient increases with increasing system size. The long‐time decay of the shear stress, heat flux, and velocity autocorrelation function is also discussed quantitatively.

A technique for the calculation of mass, energy, and momentum densities at planes in molecular dynamics simulations

We present a new technique for the evaluation of hydrodynamic densities (for example mass, energy or momentum densities) at planes in molecular dynamics simulations. This technique employs an easily computed expression for the density at a plane that is formally exact, unlike other expressions such as histogram approximations. We present simple examples of applications of this procedure to the calculation of mass and momentum densities, and hence the streaming velocity, at planes in a fluid undergoing planar Poiseuille flow, and show how the temperature profile can be obtained by the same procedure.