Ab-initio molecular orbital calculations were performed in order to examine the mechanism of proton transfer in pairs of two H2O molecules and in pairs of two C-20(OH)(2) molecules. We found that the energy barrier to the proton transfer from one oxygen atom to the other oxygen atom is governed by the distance between the two oxygen atoms. The zero-point vibrational energy (ZPVE) correction lowers and narrows this energy barrier, which leads to enhance the tunneling effect when the distance between the two oxygen atoms is around 2.6 Angstrom. (C) 2002 Published by Elsevier Science B.V.