  We investigate from first principles screening effects in endohedral
complexes when small molecules, characterized by a finite electronic dipole
moment, such as HF, LiF, NaCl, and H$_2$O, are encapsulated into different
nanoscale cages. We find that screening effects crucially depend on the nature
of the intramolecular bonds of the cage: screening is maximum in covalent-bond
carbon nanocages, while it is reduced in partially-ionic ones. Interestingly,
in the case of the ionic-bond nanocages, an antiscreening effect is observed:
due to the relative displacement of positive and negative ions, induced by the
dipole moment of the encapsulated molecule, these cages act as dipole-field
amplifiers. Our results open the way to the possibility of tuning the dipole
moment of nanocages and of generating electrostatic fields at the nanoscale
without the aid of external potentials. Moreover, we can expect some
transferability of the observed screening effects also to nanotubes and 2D
materials.
