Free energy and entropy for inserting cavities in water: Comparison of Monte Carlo simulation and scaled particle theory results

The process of inserting cavities in water is studied with the aim of a better description of some of the terms necessary in continuum quantum mechanical models. Free-energy changes for the formation of soft and hard spherical cavities in TIP4P water have been computed by Monte Carlo (MC) simulation with statistical perturbation theory, up to a radius of 6 A. The free-energy change for the formation of a hard sphere, ΔGcav, is obtained combining the ΔGsol of a soft repulsive sphere with the ΔG corresponding to the process of transforming the soft sphere into a hard one. Two definitions of hard-sphere repulsive potentials have been considered, one only based on the distance of oxygens from the center of the cavity, while the other also excludes hydrogens from the same region. Differences in free energies are significant. The cubic polynomial expression ΔGcav, obtained by extrapolating the exact scaled particle theory (SPT) expression for very small excluding cavities, gives results in agreement with MC, wi...