We applied a Quantum Mechanics/Molecular Mechanics method (QM/MM) that makes use of the mean field approximation to study the polarization of hydrogen fluoride (HF) in its liquid phase. The method is based on the calculation of the Averaged Solvent Electrostatic Potential from Molecular Dynamics data (ASEP/MD). Our model considers the HF molecule to be nonrigid, the H-F bond length can vary, and includes the effect of the electron correlation calculated at the Moller-Plesset second-order (MP2) level and the effect of the solvent polarization. The H-F bond elongates and undergoes strong polarization when it passes from the gas to the liquid phase. The ASEP/MD method provides an adequate description of this polarization, and reproduces adequately both the thermodynamics and the structure of the liquid. A comparison between the performances of two-site and three-site models for the HF molecule is also presented.