Diffusion in a rough potential revisited

TL;DR

This study investigates how the diffusion coefficient of a Brownian particle in rugged energy landscapes depends on landscape roughness, revealing the limitations of existing models and the impact of spatial correlations and traps on diffusion behavior.

Contribution

The paper introduces a new theoretical framework accounting for three-site traps and spatial correlations, improving predictions of diffusion in rugged landscapes beyond Zwanzig's original model.

Findings

Zwanzig's expression overestimates diffusion at high ruggedness

Three-site traps significantly reduce diffusion in uncorrelated landscapes

Spatial correlations alter the effective diffusion coefficient

Abstract

Rugged energy landscapes find wide applications in diverse fields ranging from astrophysics to protein folding. We study the dependence of diffusion coefficient $(D)$ of a Brownian particle on the distribution width $(\varepsilon)$ of randomness in a Gaussian random landscape by simulations and theoretical analysis. We first show that the elegant expression of Zwanzig [PNAS, 85, 2029 (1988)] for $D(\varepsilon)$ can be reproduced exactly by using the Rosenfeld diffusion-entropy scaling relation. Our simulations show that Zwanzig's expression overestimates $D$ in an uncorrelated Gaussian random lattice - differing by almost an order of magnitude at moderately high ruggedness. The disparity originates from the presence of "three-site traps" (TST) on the landscape -- which are formed by the presence of deep minima flanked by high barriers on either side. Using mean first passage time formalism, we derive a general expression for the effective diffusion coefficient in the presence of TST, that quantitatively reproduces the simulation results and which reduces to Zwanzig's form only in the limit of infinite spatial correlation. We construct a continuous Gaussian field with inherent correlation to establish the effect of spatial correlation on random walk. The presence of TSTs at large ruggedness $(\varepsilon \gg k_{B}T)$ give rise to an apparent breakdown of ergodicity of the type often encountered in glassy liquids.