Self-consistent-field calculation of Pauli repulsion and dispersion contributions to the solvation free energy in the polarizable continuum model

By using the theory of intermolecular forces, two new expressions for Pauli repulsion and dispersion contributions to the solvation free energy are derived. These expressions contain explicitly the solute electron density and, therefore, can be used directly in the SCF calculation of the solute wave function within the polarizable continuum model (PCM). The final expressions are very simple and include also some intrinsic solvent properties which are, for repulsion, the density, the molecular weight, the number of valence electrons, and for dispersion, the refractive index and the ionization potential. This new approach does not depend on any given intermolecular potential and it can be adapted to any choice of basis set. For small-size basis sets, even minimal, the dispersion contribution is obtained in two steps and includes the effect of adding diffuse and polarization functions, not used in the wave function itself. This method has been implemented in our HONDO package, in a version which includes the cavitation contribution, determined by the Pierotti-Claverie method, and the polarization contribution determined by the Miertus-Scrocco-Tomasi method. Some preliminary results on solutes containing C, H, O, and N are presented for solvation in water, n-hexane, and l-octanol. The quality of these results, given the simplicity of the PCM, is acceptable and of great interest for future developments.