  We propose a systematic, {\em a priori} approach to the problem of the
calculation of solvation energies using continuum dielectric models coupled to
quantum mechanical description of reacting molecules. Our method does not rely
on empirically scaled van der Waals radii to create a dielectric cavity, but
rather uses the electron density of the reactants as the physical variable
describing the cavity. In addition, the precise choice of cavity is made by
ensuring that the dielectric reproduces the correct linear response of the
solvent to electrostatic perturbations. As a model application which is
interesting in its own right, we study the hydrolysis of methylene chloride, a
representative model waste compound in supercritical oxidation experiments, and
which has shown surprising solvation effects close to the critical point of
pure water (T=394$^o$ C, P=221 bar). Using our {\em a priori} methodology, we
find results in good agreement with available experimental reaction barriers.
We then study, in a controlled manner, the relative importance of various
further approximations that are routinely performed in the literature such as
the use of spherical cavities, the replacement of the reactant by a dipole, or
the neglect of self consistency in solving the electrostatic problem.
