Unidirectional diffusion of methane in AlPO4-5

Tracer diffusion of methane molecules in the unidirectional channels of AlP O4-5 has been studied by means of molecular dynamics simulations. A one-dim ensional hop-and-cross model is introduced and is shown to be able to repro duce the molecular dynamics results accurately and we profit by extensive s peedup in computational time. After elimination of system size effects by u sing the new model, two regimes can be recognized: a short-time regime wher e the mean square displacement is proportional to t(0.6), and a long-time r egime where the proportionality is linear. (C) 1999 American Institute of P hysics. [S0021-9606(99)52422-7].