  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.
