A general theory of the effect of the electronic coupling between molecules on the response of the system as a whole to an external electromagnetic field is presented. The role of the non-linear response function for various physical quantities and their Fourier transformation is discussed. The non-linear susceptibilities to any rank for a denaturated system of particles are calculated. Also the non-linear response function for a coupled system is computed. A non-linear susceptibility for the electric dipole is obtained. Methods of the linear theory are developed to obtain the total non-linear refractive index. (author). 17 refs.