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 X 10(-7) m(2)/s) and passing frequency (0.305) when a static lattice approximation is used over a full dynamic lattice (1.33 X 10(-7) m(2)/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. (C) 2000 American Institute of Physics. [S0021-9606(00)70406-5].