Energy Barriers and Activated Dynamics in a Supercooled Lennard-Jones Liquid

TL;DR

This study links the potential energy landscape topography of a Lennard-Jones supercooled liquid to its relaxation dynamics, revealing how energy barriers and metabasins influence activated hopping processes.

Contribution

It introduces a new method to accurately identify energy barriers along reaction paths and relates these barriers to dynamical measures in supercooled liquids.

Findings

Effective activation energies match computed energy barriers.

Escape from metabasins involves multi-minima sequences.

No change in diffusion mechanism around the mode-coupling transition.

Abstract

We study the relation of the potential energy landscape (PEL) topography to relaxation dynamics of a small model glass former of Lennard-Jones type. The mechanism under investigation is the hopping betweem superstructures of PEL mimima, called metabasins (MB). From the mean durations $\tauphi$ of visits to MBs, we derive effective depths of these objects by the relation $\Eapp=\d\ln\tauphi/\d\beta$, where $\beta=1/\kB T$. Since the apparent activation energies $\Eapp$ are of purely dynamical origin, we look for a quantitative relation to PEL structure. A consequence of the rugged nature of MBs is that escapes from MBs are not single hops between PEL minima, but complicated multi-minima sequences. We introduce the concept of return probabilities to the bottom of MBs in order to judge whether the attraction range of a MB was left. We then compute the energy barriers that were surmounted. These turn out to be in good agreement with the effective depths $\Eapp$, calculated from dynamics. Barriers are identified with the help of a new method, which accurately performs a descent along the ridge between two minima. A comparison to another method is given. We analyze the population of transition regions between minima, called basin borders. No indication for the mechanism of diffusion to change around the mode-coupling transition can be found. We discuss the question whether the one-dimensional reaction paths connecting two minima are relevant for the calculation of reaction rates at the temperatures under study.