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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 1-octanol. The quality of these results, given the simplicity of the PCM, is acceptable and of great interest for future developments.

We present a method for computing intermolecular energies of large molecules based on a suitable fragmentation scheme, which allows one to express the complete interaction energy as a sum of interaction energies between pairs of fragments. The main advantage consists in the possibility of using standard ab initio quantum methods to evaluate the fragment energies. For the 4-n-pentyl-4′-cyanobiphenyl (5CB) dimer, the present results indicate that the most favorite arrangement corresponds to an antiparallel side-by-side geometry with a stabilization energy of about 16 kcal/mol. It is shown that, by the present method, the interaction energy of the 5CB dimer can be evaluated for all geometrical conformations and, in principle, it can be used for bulk simulations.

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