Total and direct correlation function integrals from molecular simulation of binary systems

The possibility for obtaining derivative properties for mixtures from integrals of spatial total and direct correlation functions obtained from molecular dynamics simulations is explored. Theoretically well-supported methods are examined to extend simulation radial distribution functions to long range so the integrals can converge. A previously published method developed for pure atomic fluids is here extended to handle simulations of molecular mixtures using all-atom force fields. We first test the method on simulations of Lennard-Jones/Stockmayer mixtures and show that that the results are consistent with an excess Helmholtz energy model fitted to available simulations. In addition, simulations of water/methanol and water/t-butanol mixtures have been carried out. The method yields results for partial molar volumes, activity coefficient derivatives, and individual correlation function integrals in reasonable agreement with smoothed experimental data. The proposed method for obtaining correlation function integrals is shown to perform at least as well as or better than two previously published approaches.