Boundary conditions for molecular dynamics simulation of water transport through carbon nanotubes

We compare both new and commonly-used boundary conditions for generating pressure-driven flows through carbon nanotubes (CNTs) in molecular dynamics (MD) simulation. Three systems are considered: a finite CNT membrane with streamwise periodicity and gravity forcing; a non-periodic finite CNT membrane with reservoir pressure control; and an infinite CNT with periodicity and gravity forcing. The first system is simple to implement in common MD codes, while the second system is more complex to implement, and the selection of control parameters is less straightforward. The required level of user-input for such a system was found to be largely dependent on selection of state controllers used in the reservoirs. A large pressure difference is required across the realistic membrane system reservoirs to compensate for large pressure losses at the entrance and exit of the nanotube. Despite a dramatic increase in computational efficiency, an infinite length CNT does not account for these significant inlet and outlet effects, suggesting that a much lower pressure gradient is required to achieve a specified mass flow rate. Use of an infinite channel also restricts natural flow development through the CNT due to explicit control of the fluid. Observation of radial density profiles suggest that this results in over-constraint of the water molecules in the channel.