The effect of intramolecular coupling on the optical susceptibilities up to third order in the external field is studied using a model molecule consisting of two coupled harmonic curves of electronic energies with minima displaced in energy and nuclear coordinates. We consider two vibrational states and we include non-adiabatic effects for this two-state model. We have calculated the absorption for one-, two- and three-photon processes for values of the parameters characteristic of large organic molecules diluted in water. Our results show that intramolecular coupling has a large effect on this property. © 1994.