Improved estimates for hydration free energy obtained by the reference interaction site model

We propose to improve the existing free energy expressions obtained within the framework of the reference interaction site model (RISM) combined with the hypernetted closure. The proposed expression is based on the partial wave expression [S. Ten-no, J. Chem. Phys. 115 (2001) 3724] but includes semiempirical corrections to account properly for excluded volume and hydrogen bonding effects. Testing several free energy expressions for various polar and hydrophobic solutes, we have found that such empirical parameterization of the partial wave expression can provide accurate estimates of hydration energies for different hydrophobic and polar solutes. The proposed correction allows one to reduce the discrepancy between the experimental and the calculated data down to 0.7 kcal/mol.