The direct correlation function of a crystalline solid

TL;DR

This paper calculates the direct correlation function of a crystalline solid, revealing significant differences from liquids and implications for elastic constants and thermodynamics in solids.

Contribution

First explicit calculation of the DCF for a crystalline solid using fundamental measure theory, highlighting differences from liquid DCFs and implications for elasticity.

Findings

DCF of a crystal differs from liquid DCF in shape and magnitude

Vacancies dominate the DCF in crystals

Elastic constants derived match simulation results

Abstract

Direct correlation functions (DCFs), linked to the second functional derivative of the free energy with respect to the one-particle density, play a fundamental role in a statistical mechanics description of matter. This holds in particular for the ordered phases: DCFs contain information about the local structure including defects and encode the thermodynamic properties of crystalline solids; they open a route to the elastic constants beyond low temperature expansions. Via a numerical tour de force we have explicitly calculated for the first time the DCF of a solid: based on the fundamental measure concept we provide results for the DCF of a hard sphere crystal. We demonstrate that this function differs at coexistence significantly from its liquid counterpart - both in shape as well as in its order of magnitude - because it is dominated by vacancies. We provide evidence that the traditional use of liquid DCFs in functional Taylor expansions of the free energy is conceptually wrong and show that the emergent elastic constants are in good agreement with simulation-based results.