We study with ab initio molecular dynamics the change that the electric dip
ole moment of water molecules undergoes in passing from the gas to the liqu
id phase. Our analysis is based on the recently introduced maximally locali
zed Wannier functions and is devoid of the ambiguities that have affected p
revious attempts. We find that in the liquid the dipole moment has an avera
ge 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 distributio
n around this average value is observed. The relevance of these results for
current modeling of water is discussed. [S0031-9007(99)08956-5].