The EXP pair-potential system. IV. Isotherms, isochores, and isomorphs in the two crystalline phases

This paper studies numerically the solid phase of a system of particles interacting by the exponentially repulsive pair potential, which is a face-centered cubic (fcc) crystal at low densities and a body-centered cubic (bcc) crystal at higher densities [U. R. Pedersen et al., J. Chem. Phys. 150, 174501 (2019)]. Structure is studied via the pair-distribution function and dynamics via the velocity autocorrelation function and the phonon density of states. These quantities are evaluated along isotherms, isochores, and three isomorphs in both crystal phases. Isomorphs are traced out by integrating the density-temperature relation characterizing configurational adiabats, starting from state points in the middle of the fcc-bcc coexistence region. Good isomorph invariance of structure and dynamics is seen in both crystal phases, which is notable in view of the large density variations studied. This is consistent with the fact that the virial potential-energy correlation coefficient is close to unity in the entire fcc phase and in most of the bcc phase (basically below the re-entrant density). Our findings confirm that the isomorph theory, developed and primarily studied for liquids, applies equally well for solids.