System-Size Dependence in Grand Canonical and Canonical Ensembles

TL;DR

This paper investigates how thermodynamic potentials differ between canonical and grand canonical ensembles as a function of system size, providing exact relations and detailed analysis for a non-interacting Fermi gas, with implications for density functional theory.

Contribution

It derives exact size-dependent relations between thermodynamic potentials in canonical and grand canonical ensembles and analyzes their behavior for a simple Fermi gas model.

Findings

Exact relations between free energy and pressure for arbitrary system size.

Asymptotic equivalence of ensembles as system size becomes large.

Size dependence originates from inter-particle distance and quantum length scales.

Abstract

The thermodynamics for a system with given temperature, density, and volume is described by the Canonical ensemble. The thermodynamics for a corresponding system with the same temperature, volume, and average density is described by the Grand Canonical ensemble. In general a chosen thermodynamic potential (e.g., free energy) is different in the two cases. Their relationship is considered here as a function of the system size. Exact expressions relating the fundamental potential for each (free energy and pressure, respectively) are identified for arbitrary system size. A formal asymptotic analysis for large system size gives the expected equivalence, but without any characterization of the intermediate size dependence. More detailed evaluation is provided for the simple case of a homogeneous, non-interacting Fermi gas. In this case, the origin of size dependence arises from only two length scales, the average inter-particle distance and quantum length scale (thermal deBroglie or Fermi length). The free energies per particle calculated from each ensemble are compared for particle numbers $2\le N\le 64$ for a range of temperatures above and below the Fermi temperature. The relevance of these results for applications of density functional theory is discussed briefly.