MOLECULAR-DYNAMICS SIMULATIONS IN ZEOLITES - FROM DETERMINISTIC TO RANDOM MOTION

Molecular dynamics (MD) simulations of argon in silcalite were performed in order to study the transition from the deterministic motion at short times to the random motion at long times. A characteristic length lambda (time lambda) is associated with this change in dynamics. At a temperature T=229(14) K, we find lambda = 0.45(8) angstrom [tau = 0.21 (6) ps]. A detailed analysis of the trajectories shows that the optimal choice for the elementary time step DELTAt in MD is slightly below tau. Little is gained by decreasing DELTAt as two trajectories with almost identical initial conditions diverge exponentially fast with time. A MD algorithm with variable DELTAt is proposed and used to show that the size of the zeolite channels and cavities influences how the asymptotic Gaussian random process is reached.