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. (C) 2001 American Institute of Physics.