In the current work, we combine PIMD with the reference potential method for the first time and demonstrate its application in the study of the intramolecular proton transfer in the protonated 1,8-bis(dimethylamino)naphthalene (DMANH) molecule. The free energy profile at the PIMD/BLYP-D3/6-31G(d)/MM level of theory along the proton transfer pathway was calculated leveraging a more efficient sampling at the PM6/MM level. A 545-fold enhancement in the computational efficiency was achieved as compared to a direct PIMD simulation at the same ab initio level of theory.