Evaluation of dispersion-repulsion contributions to the solvation energy. Calibration of the uniform approximation with the aiad of RISM calculations

The dispersion-repulsion contributions to the solvation energy, computed with surface integrals and uniform approximation for some hydrocarbons in water and methanol, are compared with the results obtained using more realistic solvent distribution functions from RISM integral equation. The cavity surface we use is not spherical and models the structure of the enclosed solute molecule through a set of interlocking spheres centered on the interaction sites. The RISM radial distribution functions are used to fix the value of the radii of closest approach between solute-solvent interaction centers that define a minimal cavity surface. Two ways of working with this approximation are examined comparing also local contributions on the cavity surface. It is possible to find a set of cavity radii, to be used in the uniform approximation, that lead to dispersion-repulsion contributions in good agreement with those derived from RISM radial distribution functions and show a satisfactory degree of transferability.