Atomic pair correlations in Lennard-Jones fluids: Analysis in terms of structural diffusion models

Abstract

We present an analysis of pair correlation data of a Lennard-Jones fluid (argon), covering a wide range of thermodynamic states, in terms of structural diffusion (SD) models. These are based on the notion of a locally observed ordered structure and of spatial disorder as an outcome of a random spatial process. A general recipe is developed for analysing structural data by such models. First, we analyse fluid argon in terms of the simple SD model (SDM1). Nine lattices are chosen for the description of the local order in the fluid. It is found that close packed lattices are better suited for describing the high density fluid structure while open lattices are better suited for the description of the low density fluid. The SDM parameters obtained by constrained optimization describe the dependence of the pair correlation on density and temperature, and constitute a structural map of the fluid in the (ρ, T) plane. Two possible ways are presented for improving the fitting of g(r) data: (1) distortion of a chosen symmetric lattice (FCC in our case) by varying the basic cell angles and optimizing within the SDM1. It is found that the higher symmetry lattices, BCC and FCC, give the best fitting to the experimental g(r); (2) use of an extended version of the model (SDM2) that enables us to obtain a refined fitting through a built-in contraction function, f(r). A simple choice for f(r) in the model expression of g(r), applied to the four most dense lattices, gives an improved fit for high densities. The best results are obtained with the hcp lattice.