Water Molecule Dipole in the Gas and in the Liquid Phase

We study with ab initio molecular dynamics the change that the electric dipole moment of water molecules undergoes in passing from the gas to the liquid phase. Our analysis is based on the recently introduced maximally localized Wannier functions and is devoid of the ambiguities that have affected previous attempts. We find that in the liquid the dipole moment has an average value of about 3 D, 60% higher than in the gas phase. This value is much larger than is currently assumed (2.6 D). Furthermore, a broad distribution around this average value is observed. The relevance of these results for current modeling of water is discussed.