  The dielectric properties of molecules or nanostructures are usually modified
in a complex manner, when assembled into a condensed phase. We propose a
first-principles method to compute polarizabilities of sub-entities of solids
and liquids, which accounts for multipolar interactions at all orders, and is
applicable to any semiconductor or insulator. The method only requires the
evaluation of induced fields in the condensed phase, with no need of multiple
calculations for each constituent. As an example, we present results for the
molecular polarizabilities of water in a wide pressure and 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 the Clausius-Mossotti relation is not valid.
