The effect of coupling between levels in quantum wells or quantum dots is described in Green's function formalism. The structure eigenvalues are shown to have a Brillouin-Wigner continued-fraction expression that allows to give a general and intuitive meaning to levels coupling, described in terms of an off-diagonal self-energy. The concept of coupling is linked to a general potential matrix and can be given the same mathematical form for all kinds of coupling (inter-and intra-quantum dot and

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... antum well), in which off-diagonal self-energy contributions assume each time a different conceptual meaning. Furthermore, the same scheme, based on off-diagonal self-energies, allows to evaluate renormalization contribution due to each structure energy level in a natural and easy way.