How to reconcile non-equlibrium with equilibrium measurements of slip length

We show that slip measurements in molecular dynamics simulations using non-equilibrium and equilibrium (Green-Kubo) methods can be reconciled by properly accounting for the hydrodynamic wall location. The latter is an integral part of the Green-Kubo (GK) formulation and denotes the location at which the slip boundary condition is to be applied, typically inside the fluid. In contrast, non-equilibrium methods measure the slip at the fluid-solid interface. Although GK expressions for calculating the hydrodynamic wall location have been developed, statistical noise as well as the lack of a well-defined plateau in the associated GK integral has made them imprecise and cumbersome to use, and as a result, led to their neglect. Unfortunately, the distance between the actual and the hydrodynamic wall location can be sufficiently large that if their difference is not taken into account, the equilibrium and non-equilibrium measurements appear to diverge. Here we propose an alternative approach for calculating the hydrodynamic wall location that is not subject to the above limitations. Extensive molecular dynamics simulations are used to validate our approach.