Structure and thermodynamics for some small polar interaction-site fluids

Abstract

Thermodynamic properties of a number of small polar interaction-site models have been estimated using Reference Interaction Site Model (RISM), with hypernetted chain (HNC) closure for structure calculations. These models include the statistical process control (SPC) and TIPS models for water as well as a new model proposed in this work. The estimated properties are compared with experimental structure data as well as experimentally measured thermodynamic data for real fluids. For water, both SPC and TIPS predict values of residual energy that are significantly below the experimental values. The estimated structures for these models are also phase shifted with respect to recent neutron data. Pair-correlation functions for the new model are in better agreement with experimental neutron data, as well with energy data. Canonical molecular dynamics simulations of the SPC model and the new model indicate that the estimated structure from these simulations for the SPC model is in closer agreement with neutron data than the new model. Residual energies from the TIPS model for methanol are also underpredicted from RISM and HNC closure. Recalculation of the TIPS parameters to corresponding Lennard—Jones parameters and the adoption of the hydrogen core from the new water model brings the estimated energies closer to the experimental values. New models for H 2S and methylchloride needs further refinement and additional evaluations against results from molecular simulations. Comparisons of results for the new models of water and methanol indicate that a dielectric correction to the RISM equation induces minor changes to the residual energies for these components. Numerical solutions to the structure of a model system consisting of water and methanol in the dielectrically corrected RISM version is also presented.