Application of transient-time correlation functions to nonequilibrium molecular-dynamics simulations of elongational flow

The transient-time correlation function (TTCF) technique of Morriss and Evans [Mel. Phys. 54, 629 (1985); Phys. Rev. A 35, 792 (1987); Mol. Phys. 61, 1151 (1987); Phys. Rev. A 38, 4142 (1988); Statistical Mechanics of Nonequilibrium Liquids (Academic, London, 1990)] is applied to the case of an atomic fluid undergoing steady isothermal elongational flow. It is found that nonequilibrium molecular-dynamics TTCF calculations of the diagonal elements of the pressure tensor are extremely efficient for small applied strain rates, where the signal-to-noise ratio for the equivalent direct time-averaged pressures is far too low. At higher strain rates, TTCF is seen to faithfully reproduce the long-time steady-state values of the pressures, but is unable to account for observed transient oscillations. The technique thus provides an unambiguous means of calculating the long-time steady-state response of a fluid under steady elongational flow and opens the possibility of studying more complex molecular fluids under relatively weak flow, allowing for greater simulation time compared to the relaxation time of the fluid.