A set of atomic radii used for the construction of solute cavities in the framework of the polarizable continuum model (PCM) is extended and validated with the aim of supporting the investigation of lanthanide(III) complexes in aqueous solution. The parameterization of the atomic radii for the whole Ln(III) series is performed by minimizing the differences between the experimental and the calculated standard hydration free energies of the ions calculated at the HF level. The optimized radii show a remarkable linear relationship with effective ionic radii and well reproduce the experimental hydration free energies also when electron correlation effects are included in the calculations. We have next validated a mixed discrete continuum model in which a supermolecule formed by the ion and by water molecules in the first hydration shell is immersed in a polarizable continuum. The molecular structures, the relative stability of the octa- with respect to the nonahydrated species, and the ion hydration free energies have been calculated for the neodymium(III) and ytterbium(III) aqueous ions. Results are in agreement with experimental evidence, both from structural and energetic standpoints. The molecular structures optimized including surrounding effects are in better agreement with the experimental structures than the in vacuo geometries. Moreover, the results show that the energetic properties of these systems in aqueous solution can be effectively calculated by using the structures optimized in vacuo, and including correlation effects in the gas-phase reaction of complex formation.