(167d) Limitations and Recommendations for the Calculation of Shear Viscosity Using Reverse Nonequilibrium Molecular Dynamics

The reverse nonequilibrium molecular dynamics (RNEMD) method (Muller-Plathe, Phys Rev 59, 1999) calculates the shear viscosity of a fluid by imposing a non-physical exchange of momentum and measuring the resulting shear velocity gradient. In contrast with most other nonequilibrium methods, because linear momentum and energy can be conserved in RNEMD, coupling to an external thermostat is not strictly required. RNEMD has been used to calculate the viscosities of fluids with complexities varying from simple atoms to complex molecules. It is known that RNEMD will fail to yield linear shear velocity profiles at high momentum flux rates. In this study we investigate the range of momentum flux values over which RNEMD yields usable (linear) velocity gradients. We find that non-linear velocity profiles result primarily from gradients in fluid temperature and density. The temperature gradient results from conversion of heat into bulk kinetic energy, which is transformed back into heat elsewhere via viscous heating. An expression is derived to predict the temperature profile resulting from a specified momentum flux for a given fluid and simulation cell. Although primarily bounded above, we also describe milder low-flux limitations. RNEMD results for a Lennard-Jones fluid agree with equilibrium molecular dynamics and conventional nonequilibrium molecular dynamics calculations at low shear, but RNEMD under-predicts viscosity relative to conventional NEMD at high shear.