Ab initio Monte Carlo computations were carried out on H₂O dimer system. By introducing the energy partitioning scheme that we have developed recently, ab initio calculated H₂O-H₂O interaction can be analyzed from the viewpoint of atom-atom interaction. The electronic polarization caused by the interaction and its temperature dependence are also discussed. To our best knowledge, this is the first report on the thermal distribution of electronic distortion energy assigned to a molecule.