(27c) Molecular Simulation Of Transport In Nanopores

In nonequilibrium molecular dynamics (NEMD) simulations, transport properties are determined by computing the average of the response over the steady state. However, the steady-state average becomes very noisy for realistic (i.e. of the order of those used in experiments) external fields or shear rates. Having a large signal-to-noise ratio - and hence subjecting the fluid to large shear rates - is therefore crucial to obtain meaningful steady-state averages. For instance, the lowest shear rates accessible by molecular dynamics are at least 4 orders of magnitude larger than those typically used in experiments. Current NEMD methods are therefore unable to shed light on a number of recent experimental measurements on films of about 5-8 molecular layers.

We address the inability of current MD methods to study nanoconfined liquids subjected to realistic external fields (or shear rates). We apply a nonlinear generalization of the Green-Kubo relations, the so-called transient time correlation function (TTCF) formalism. TTCF gives an exact relation between the nonlinear steady-state response and the so-called transient time correlation function. We demonstrate that this approach allows to study the transport properties of a nanoconfined fluid.