Abstract
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.
Highlights
The EXP pair potential is the purely repulsive exponentially decaying function, vEXP(r) = ε e−r/σ . (1)The system of particles interacting via this pair potential has been studied much less than, e.g., the Lennard-Jones or inverse power-law (IPL) pair-potential systems
The EXP pair potential may be regarded as the “mother of all pair potentials” in the sense that it explains the quasiuniversality of the structure and dynamics of simple liquids that applies for a large class of pair potentials
All simulations were carried out using the double-precision version of the Roskilde University Molecular Dynamics (RUMD) code, which is optimized for graphics-processing-unit (GPU) computing
Summary
Any pair potential, which can be written as a finite sum of EXP terms with coefficients much larger than kBT, defines a system in the same quasiuniversality class as the hard-sphere and Lennard-Jones systems.. Paper I18 provided an example of quasiuniversality by showing that the radial distribution function (RDF) of the Lennard-Jones system is close to that of the EXP system at state points where the two systems have the same reduced diffusion coefficient. There is a re-entrant liquid phase at the highest densities studied, which are of order unity in the “EXP unit system” defined by σ and ε in Eq (1) This means that above a certain temperature there is no stable solid phase, a property the EXP system has in common with other pair-potential systems such as the Gaussian-core model, which do not have a diverging energy at zero particle separation. The line separating fcc and bcc state points found from simulations is full red; its extrapolation to the zero-temperature coexistence density is dashed
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