Accuracy of Solvation Free Energy Estimated from Integral Equation Theory for Two-Component System of Lennard-Jones Fluid

Solvation of complex molecules has attracted many researchers. Typically they employ molecular dynamics (MD) simulations to calculate solvation free energy (SFE), whereas there is another route to calculate SFE: i.e., integral equation theory (IET). One of the IETs to calculate SFE for molecular liquid is three-dimensional reference interaction site model (3D-RISM) theory. 3D-RISM can calculate SFE with cheaper computer cost than MD, which is one of the advantages of the theory. Though 3D-RISM significantly improved the description of solvation in comparison with a traditional IET, the accuracy of SFE evaluated from the theory was still unsatisfactory. Our final goal is to develop a new theoretical tool to calculate SFE with satisfactory accuracy and relatively cheap computer cost. As a first step, this study examines how IET is accurate or inaccurate in terms of SFE. For simplicity, we choose Lennard-Jones systems composed of two components (i.e. solute and solvent). In particular, we focus on two approximations used in IET, i.e., hypernetted chain and Kovalenko-Hirata approximations We discuss the accuracy of these approximations by comparing SFEs evaluated from the corresponding approximations with those from MD simulations.