Direct correlation functions and the density functional theory of polar solvents

To describe the solvation properties of polar or charged species, and the reaction free-energy of charge transfers, we propose a generalization of the Marcus electrostatic polarization free-energy functional which accounts for the molecular nature of the solvent. The proposed generic free energy functional relies crucially on the knowledge of the direct correlation function of the homogeneous solvent. For the case of a dipolar solvent, we show how this fundamental quantity can be extracted directly from molecular dynamics simulations of the pure solvent instead of the traditional route of integral equation theories. The direct correlation function computed from simulation is compared to approximate ones obtained from linearized and quadratic HNC integral closures. The performance of the corresponding density functionals is assessed, in comparison to “exact” molecular dynamics results, for the solvation free-energies and the solvent density profiles around simple molecular solutes.