We compared the performances of two methods that combine quantum mechanics and molecular mechanics for the study of liquid systems. They differ in the description of the solute-solvent interaction. One makes use of the mean field approximation and the other does not. We show that the introduction of this approximation does not introduce significant errors into the induced dipole moment of the solute, the interaction energy or the solvent structure, while it permits a considerable reduction (from several thousand to four or five) of the number of quantum calculations and hence of the computational demands.