  We investigate dielectric saturation and increment in polar liquids under
external fields. We couple a previously introduced dipolar solvent model to a
uniform electric field and derive the electrostatic kernel of interacting
dipoles. This procedure allows an unambiguous definition of the liquid
dielectric permittivity embodying non-linear dielectric response and
correlation effects.We find that the presence of the external field results in
a dielectric anisotropy characterized by a two-component dielectric
permittivity tensor. The increase of the electric field amplifies the
permittivity component parallel to the field direction, i.e. dielectric
increment is observed along the field. However, the perpendicular component is
lowered below the physiological permittivity, indicating dielectric saturation
perpendicular to the field. By comparison with Molecular Dynamics simulations
from the literature, we show that the mean-field level dielectric response
theory underestimates dielectric saturation. The inclusion of dipolar
correlations at the weak-coupling level intensify the mean-field level
dielectric saturation and improves the agreement with simulation data at weak
electric fields. The correlation-corrected theory predicts as well the presence
of a metastable configuration corresponding to the antiparallel alignment of
dipoles with the field. This prediction can be verified by solvent-explicit
simulations where solvent molecules are expected to be trapped transiently in
this metastable state.
