A study on the extension of correlation functions obtained from molecular dynamics simulations by the Ornstein-Zernike theory for modeled molten salts

We extend the correlation functions obtained by molecular dynamics (MD) simulation for a molten salt modeled as a superposition of the Lennard-Jones (LJ) and Coulomb potentials using the hybrid closure method, which employs the Ornstein-Zernike (OZ) theory coupled with a closure relation. An appropriate distance for switching the short-range MD part and the long-range OZ part is determined by monitoring the isothermal compressibility, excess internal energy, and pressure. The Kobryn-Gusarov-Kovalenko (KGK) closure relation is mainly employed for the hybrid closure method (MD-KGK hybrid closure). The hybrid closure with either the hypernetted chain (HNC) or Kovalenko-Hirata (KH) closure was also tested to confirm that the performance was almost equivalent to one another among the MD-HNC, MD-KH, and MD-KGK methods. The bridge function for the model molten salt is extracted using the MD-KGK hybrid closure method. At a high-density state, the bridge function shows a steep increase in the repulsive core region, as is often observed for simple fluids, whereas when the density is relatively low, the bridge function for the cation-anion pair shows a downward-sloping behavior. Furthermore, the accuracies of excess internal energy, pressure, and isothermal compressibility were also examined for the HNC, KH, and KGK approximations. For molten salt systems, these approximations exhibited a similar behavior to those for monatomic LJ fluids, especially in the high-density state. The analysis of the integrand for excess internal energy and pressure is also discussed.