A novel, self-consistent approach, applicable both for ground and excited electronic states, is introduced to calculate molecular properties in clusters and liquids. Using the method, carried out here at the second-order Moller-Plesset perturbation theory (MP2) level, we obtain an average dipole moment of 2.65 D for water in liquid. Significant changes in quadrupole moment and polarizability, due to surrounding molecules, are also found along the water plane in the direction perpendicular to the axis bisecting the H-O-H bond angle.