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

Marcus et al (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.