Optimizing free energy functionals in integral equation theories

Abstract

For the calculation of solvation thermodynamics and structures the reference interaction site model RISM is a theory with rising importance. RISM is a statistical solvent model that belongs to implicit solvation models. A central property in RISM is the bridge function which heavily influences the quality of the calculations. This work is about improvement of the accuracy of solvation thermodynamics properties calculated with RISM. The influence of the bridge function on the excess chemical potential is studied and findings are used to build a widely usable correction for RISM calculated free energies. Alchemical transition free energies of simple fluids were calculated with high accuracy using with closed form bridge dependent expressions for the excess chemical potential. The partial molar volume can be used to approximate the bridge dependent functional for small molecules in aqueous solution. This works well for neutral and charged molecules. This correction is competitive with other state of the art implicit solvent models especially in the treatment of ions.