[1][2] These relations link the macroscopic quantities such as the refractive index to the dipole polarization of the individual atoms or molecules.
Similarly for the pion, the lightest meson and the carrier of the long range part of the nuclear force, its typical non-relativistic scattering for individual nucleons has a dominant dipole structure with a known average dipole polarizability of strength ("the average scattering volume").
The physics becomes closely similar although the nuclear density is about 15 orders of magnitude larger than that of ordinary matter and the nature of the dipole interaction is totally different.
This is the reason why in the molecular case of the classical Lorentz-Lorenz effect so many incompatible derivations give the same result.
[5] The EELL correction was first applied to the line shifts of hydrogen-like atoms, where the electron in the Coulomb field is replaced by a negatively charged pion.