Multipole interactions in the molecular polarizabilities of water from ambient to high pressures*

Knowledge of molecular polarizabilities in condensed phases provides important information about molecular crystals, and in general about materials composed of molecular or nano-building blocks. We propose a first-principles method based on electronic densities to compute molecular polarizabilities in condensed phases. The method includes all multipole interactions in addition to the dipole-dipole one, and it is applicable to any semiconductor or insulator. We present results for molecular polarizabilities of liquid water in a wide pressure-temperature range. We found that at ambient conditions, the dipole-induced-dipole approximation is sufficiently accurate and the Clausius-Mossotti relation may be used, e.g. to obtain molecular polarizabilities from experimental refractive indexes. However with increasing pressure this approximation becomes unreliable and in the case of ice X, where covalent bonds are present, the dipole-induced-dipole approximation breaks down.