Extraction of site-site bridge functions and effective pair potentials from simulations of polar molecular liquids

We develop an efficient method to extract site-site bridge functions from molecular simulations. The method is based on the inverse solution of the reference site interaction model. Using the exact long-range asymptotics of site-site direct correlation functions defined by the site-site Ornstein-Zernike equations, we regularize the ill-posed inverse problem, and then calculate site-site bridge functions and effective pair potentials for ambient water, methanol, and ethanol. We have tested the proposed algorithm and checked its performance. Our study has revealed various peculiarities of the site-site bridge functions, such as long-range behavior, strong dependence on the electrostatic interactions. Using the obtained data, we have calculated thermodynamic properties of the solvents, namely, isothermal compressibility, internal energy, and Kirkwood-Buff integrals. The obtained values are in excellent agreement not only with molecular simulations but also with available experimental data. Further extensions of the method are discussed.