Some aspects of solvation

Some traditional approximation techniques are reviewed, concerning the effect of inserting a localized particle in an initially uniform fluid. They are put in a dynamical context, valid for both classical and quantum systems. The above picture, aimed at simple fluids, is generalized to include internal degrees of freedom. Both density profile and solvation free energy are treated in this fashion, in which a suitably defined frequency spectrum for the solvent plays a principal role.