Accuracy of some useful closure relations in combination with the reference interaction site model theory for fluids of single component diatomic molecules

We examined the accuracy of the reference interaction site model theory in terms of the thermodynamics of fluids consisting of single-component diatomic molecules, where we evaluated the internal energy, pressure, and isothermal compressibility. Two routes were tested for pressure. One is the differentiation of the Helmholtz free energy with respect to the density, and the other is the virial formula. We focused on hypernetted chain (HNC), Kovalenko–Hirata (KH), and Kobryn–Gusarov–Kovalenko (KGK) closure approximations. The most accurate closure relationship among the three is the KGK closure. The HNC and KH closures tend to overestimate the internal energy and pressure from the density derivative of the Helmholtz free energy. Moreover, it was found that the HNC closure significantly overestimated the isothermal compressibility, particularly at low density, or for molecular models with short bond lengths. The applicability of the sigma enlarging bridge (SEB) correction was also tested. After applying the SEB correction to the KH (SEB-KH) closure, a significant improvement was observed in the internal energy even for the model including the Coulomb potential.