Extension of the correlation functions of molecular dynamics simulations using the reference interaction site model theory for diatomic molecules

The correlation functions obtained by molecular dynamics (MD) simulations for diatomic molecules were extended using the reference interaction site model (RISM) theory. The Lennard-Jones (LJ) and Coulomb potentials were employed as interaction potentials. For short-range, the radial distribution function was described by the MD simulation, whereas for long-range, the correlation functions were defined by the RISM theory coupled with some closure relations. A switching function was applied to the region of connecting these two methods. By monitoring the thermodynamic quantities including the isothermal compressibility, an appropriate distance for connecting the two methods was determined. The Kobryn–Gusarov–Kovalenko closure was mainly used to couple with the RISM theory for a long-range part. The performances of the hypernetted chain and Kovalenko–Hirata (KH) closures were also examined in extending the correlation functions. The bridge functions for the RISM theory on the diatomic molecules were extracted using the combined method between the MD and RISM. The behavior of the bridge function was discussed by comparing it with that of simple monatomic fluids, such as the LJ fluids. For heteronuclear polar diatomic molecules that have partial charges, the order of the magnitude of the bridge functions was somewhat similar to that for the molten salt.