These values indicate that (i) The inclusion permittivity is significantly larger when embedded in a matrix relative to its permittivity in vacuum (ε NP ∼ 13, see Sec. 3.1 and Fig. ), and (ii) The interfacial MgO layer is much more polarizable than bulk MgO. The enhanced nanoparticle permittivity is largely due to an additional ionic polarizability component that is present in silver inclusions but absent in gas phase silver particles: When embedded in MgO, the silver atoms develop nontrivial positive Born Effective Charges , and the reorganization of these silvers and the interfacial MgO ions in the presence of an electric field contributes to the overall polarization of the system. In contrast, spherical silver particles in the gas phase only have an electronic component to their polarization.