A critical comparison of equilibrium, non-equilibrium and boundary-driven molecular dynamics techniques for studying transport in microporous materials

Transport in an idealized model with variable pore diameter as well as an AlPO4-5 zeolite is examined using three different molecular dynamics techniques: (1) equilibrium molecular dynamics (EMD); (2) external field nonequilibrium molecular dynamics (EF–NEMD); and (3) dual control volume grand canonical molecular dynamics (DCV–GCMD). The EMD and EF–NEMD methods yield identical transport coefficients for all the systems studied. The transport coefficients calculated using the DCV–GCMD method, however, tend to be lower than those obtained from the EMD and EF–NEMD methods unless a large ratio of stochastic to dynamic moves is used for each control volume, and a streaming velocity is added to all inserted molecules. Through development and application of a combined reaction–diffusion–convection model, this discrepancy is shown to be due to spurious mass and momentum transfers caused by the control volume equilibration procedure. This shortcoming can be remedied with a proper choice of streaming velocity in conjunction with a well-maintained external field, but the associated overhead makes it much less efficient than either the EMD or EF–NEMD techniques.