Two- and N-step correlated models for the analysis of molecular dynamics trajectories of linear molecules in silicalite

Recent molecular dynamics data on the diffusion of linear diatomic and tria tomic molecules in the zeolite silicalite are analyzed in terms of a new co rrelated model [F. Jousse, S. M. Auerbach, and D. P. Vercauteren, J. Chem. Phys. 112, 1531 (2000)] capable to account for both first- and higher-order correlation effects. This "N-step" model reproduces very well our calculat ed mean square displacements and diffusion coefficients of the molecules co nsidered. The improvements with respect to the results obtained with our pr evious "two-step" model [P. Demontis, J. Karger, G. B. Suffritti, and A. Ti locca, Phys. Chem. Chem. Phys. 2, 1455 (2000)] are remarkable for all molec ules except chlorine, showing that only in this case the effect of (negativ e) correlations spanning more than two jumps between channel intersections (similar to 20 Angstrom) can be neglected. The basic trajectory analysis in terms of single- and two-step models, besides being an useful reference, p rovides all the input data needed for the application of the N-step model. Indeed, in its silicalite formulation, the N-step model is strongly linked to the two-step one because it calculates the probability of a sequence of jumps in the same channel by means of the correlations between any two cons ecutive jumps. Finally, the possibility to obtain qualitative insight into the diffusive mechanism through various kind of correlation coefficients is discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)70741-0] .