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This automatically takes into account the proper coupling between the polarization and the electromagnetic field. The microscopic approach to take into account polariton effects re-expresses the exciton–exciton interactions in terms of polariton–polariton interactions by using the transformation between excitons and polaritons [60,91]. The nonlinear response is then described as scattering of polaritons on each other. For instance, exciton fusion terms translate into polariton fusion terms. In this approach, nonlinear optical processes in a bulk crystal are characterized by the excitation of polariton states at the boundary of the crystal and the possible mixing of these polaritons through their mutual interaction [60,91].

Finally, the physical meaning of including double (de-)excitation terms is worth mentioning. Let is assume that Jnm is given by the interaction between the transition dipoles of molecules m and n (cf. Eq. (6)), then the correction to the ground state energy of a pair of molecules derives from a virtual double excitation of the pair from its ground state due to the dipolar interactions, followed by its falling back to the ground state. This is the origin of the Van der Waals interaction. , as the molecular polarizability.

2 as the ones responsible for the gas-condensed matter shifts Dn of the molecular levels. It has been shown, however, that the Lorentz–Lorenz formula can be generalized to include these effects [94]. Following Ref. [94], we will in this section employ the local-field method to discuss the effect of mixing of molecular configurations and the spectra of impurities. In addition, we will apply it to calculate the dielectric tensor of anisotropic molecular crystals of complex structure. Consider a crystal of which the unit cell contains S identical molecules, labeled s = 1, 2, .