Comment on "On the importance of the free energy for elasticity under pressure"

TL;DR

This paper critiques a claim that Gibbs free energy is essential for calculating equilibrium structures and elastic constants under pressure, arguing that the statements made by Marcus et al. are incorrect.

Contribution

It clarifies misconceptions about the roles of Gibbs free energy and internal energy in determining equilibrium and elastic properties under pressure.

Findings

Marcus et al.'s claims are incorrect

Equilibrium structures can be derived from internal energy

Elastic constants are not solely determined by Gibbs free energy

Abstract

Marcus et al. (Marcus P, Ma H and Qiu S L 2002 J. Phys.: Condens. Matter 14 L525) claim that thermodynamic properties of materials under pressure must be computed using the Gibbs free energy $G$, rather than the internal energy $E$. Marcus et al. state that ``The minima of $G$, but not of $E$, give the equilibrium structure; the second derivatives of $G$, but not of $E$, with respect to strains at the equilibrium structure give the equilibrium elastic constants.'' Both statements are incorrect.