Instantaneous Normal Mode Analysis of the Levitation Effect in Zeolites

Abstract

An instantaneous normal mode (INM) analysis of the levitation effect in zeolites is presented in this paper. Lennard-Jones sorbates of variable size and polarizability diffusing in Na−Y zeolite are studied using microcanonical molecular dynamics (MD) simulations. A comparison of the dynamical information from the MD simulations with predictions based on INM analysis shows that the INM spectrum carries several striking signatures of the levitation effect. The fraction of imaginary modes mirrors the trend in the diffusion coefficient as a function of sorbate size and shows an anomalous levitation peak. The Einstein frequency, as a function of sorbate size, shows a minimum at the position of the anomalous peak. The qualitative shape of the INM spectrum changes in the anomalous regime, reflecting the availability of 12-ring window sites for adsorption, in addition to the α-cage sites. The velocity autocorrelation functions of the sorbate are well-reproduced from INM data for short time scales of one picosecon...