The effects of a dynamic lattice on methane self-diffusivity calculations in AlPO₄-5

Canonical ensemble molecular dynamics simulations were conducted for methane diffusion in AlPO₄-5 in order to assess the role of the lattice motion on adsorbate diffusivity in straight pore zeolites. Both a static lattice model and a full dynamic lattice model were used at a loading of 1.5 methane/unit cell at 295 K. Although recent simulation work has asserted that there should be a difference, we show that there is little difference in the observed methane diffusivity (1.26 × 10^-7 m²/s) and passing frequency (0.305) when a static lattice approximation is used over a full dynamic lattice (1.33 × 10^-7 m²/s and 0.328). Furthermore, we introduce a methodology for handling lattice motion in molecular simulations by utilizing the normal vibrational modes in a harmonic crystal approximation.