Dynamic Pair Correlations and Superadiabatic Forces in a Dense Brownian Liquid

TL;DR

This paper investigates the dynamic two-body correlations in a dense Brownian liquid through simulations, revealing how correlation shells decay, the roles of adiabatic and superadiabatic forces, and detailed collision dynamics.

Contribution

It introduces a division of the van Hove current into adiabatic and superadiabatic parts, providing new insights into the microscopic dynamics of dense liquids.

Findings

Outer correlation shells remain stable over time.

Adiabatic van Hove current exceeds total current, indicating overestimation by density functional theory.

Superadiabatic current opposes the total current, revealing complex force contributions.

Abstract

We study dynamic two-body correlation functions, i.e. the two-body density, the current-density correlator or van Hove current, and the current-current correlator in Brownian dynamics computer simulations of a dense Lennard-Jones bulk liquid. The dynamic decay of the correlation shells of the two-body density is examined in detail. Inner correlation shells decay faster than outer correlation shells, whereas outer correlation shells remain stable for increasing times. Within a dynamic test particle picture the mechanism is assumed to be triggered by the dislocation of the self particle, which releases the confinement of the surrounding correlation shells. We present a division of the van Hove current into an adiabatic and a superadiabatic contribution. The magnitude of the adiabatic van Hove current is found to exceed that of the total van Hove current, which is consistent with dynamic density functional theory overestimating the speed of the dynamics. The direction of the superadiabatic van Hove current opposes that of the total van Hove current. The current-current correlator reveals detailed insight in the collisions of the particles. We find a large static nearest-neighbor peak, which results from colliding particles and different dynamic peaks, that are attributed to consecutive collisions.