  The influence of intrinsic Fe defects in FeO (either single cation vacancies
or prototypical 4:1 vacancy clusters) on the electronic and dielectric
properties is studied within the density functional theory. The importance of
local Coulomb interactions at Fe atoms is highlighted and shown to be
responsible for the observed insulating Mott gap in FeO which is reduced by the
presence of defects. We investigate nonstoichiometric configurations of Fe1-xO
with x ranging from 3 to 9 % and find the aliovalent Fe cations in both the
regular and interstitial lattice sites of the considered configurations.
Furthermore, we show that the trivalent Fe ions are induced by both isolated
and clustered Fe-vacancies and introduce the empty band states inside the
insulating gap, which decreases monotonically with increasing cation vacancy
concentration. The Fe1-xO systems with high defect content become metallic for
small values of the Coulomb interaction U, yielding the increase in the
dielectric functions and optical reflectivity at low energies in agreement with
the experimental data. Due to the crystal defects, the infrared-active
transverse optic phonons split and distribute over a wide range of frequencies
clarifying the origin of the exceptionally large spectral linewidths of the
dielectric loss functions observed for wustite in recent experiments.
