Hidden scale invariance at high pressures in gold and five other face-centered-cubic metal crystals

Recent density functional theory simulations showed that metals have a hitherto overlooked symmetry termed
“hidden scale invariance” [Hummel et al., Phys. Rev. B 92, 174116 (2015)]. This scaling property implies
the existence of lines in the thermodynamic phase diagram, so-called isomorphs, along which structure and
dynamics are invariant to a good approximation when given in properly reduced units. This means that the phase
diagram becomes effectively one-dimensional with regard to several physical properties. This paper investigates
consequences and implications of the isomorph theory in six metallic crystals: Au, Ni, Cu, Pd, Ag, and Pt. The
data are obtained from molecular dynamics simulations employing many-body effective medium theory (EMT)
to model the atomic interactions realistically. We test the predictions from isomorph theory for structure and
dynamics by means of the radial distribution and the velocity autocorrelation functions, as well as the prediction
of instantaneous equilibration after a jump between two isomorphic state points. Many properties of crystals tend
to be dominated by defects, and many of the properties associated with these defects are expected to be isomorph
invariant as well. This is investigated in this paper for the case of vacancy diffusion. In regard to the perfect crystal
properties, we find the predicted invariance of structure and also, though less perfectly, of dynamics. We show
results on the variation of the density-scaling exponent γ , which can be related to the Grüneisen parameter, for all
six metals.We consider large density changes up to a factor of two, corresponding to very high pressures. Unlike
systems modeled using the Lennard-Jones potential where the density-scaling exponent γ is almost constant,
this quantity varies substantially when using the EMT potential and is also strongly material dependent.