A practical ab initio quantum-mechanical approach for calculations of free energies of molecules in solutions is developed. This approach treats the solute molecules by an explicit ab initio self-consistent-field approach while representing the solvent molecules by a pseudopotential. The solvation energies are evaluated by a free-energy perturbation approach that uses the distribution function associated with a classical force field as a reference state for the quantum-mechanical calculations. The performance of the method is examined by evaluating the solvation energy of an Li+ ion. It is found that the calculation times are not much longer than that of the corresponding classical free-energy perturbation calculations.