The effect of density on the inherent structure in liquids

Abstract

We have previously shown that diffusion and fluid flow within a liquid may be interpreted as transitions between mechanically stable arrangements of the molecules, i.e., local minima in the potential energy for the system as a whole. In addition, our previous work has shown that the equilibrium thermodynamic properties of the liquid and solid can be rigorously and usefully explained in terms of vibrational excitations within, and shifting equilibrium between, these stable molecular packings. Previously, we have determined transition rates between various packings as a function of temperature, system size, and extent of supercooling. In the present work, we study the effect of density on both the number and distribution of packing structures. Molecular dynamics simulations were carried out for systems of 32 and 108 argon-like particles at three densities. The primary obsrvations and conclusions are: (1) as the density decreases the total number of different amorphous packing structures increases; (2) the density at which the amorphous material has its minimal average energy is about 3% lower than the density at which the crystalline material has its minimal energy; (3) the activation energy to self-diffusion is found to increase with increasing density.