b) Collective model:

Within a collective model (eg., vibrational, rotational,...) excitations are interpreted in terms of the deformation of the charge or mass distribution of the nucleus.

In the previous example, the coupling terms $U_{\alpha\alpha'}$ appearing in the coupled equations (12) are determined within a cluster model. In some systems, the excitation spectrum is better described within a collective model. The excitation of the nucleus is then described in terms of deformed potentials. These can be expressed as a change in the radius at which we evaluate the optical potentials, the change depending on the relative orientations of the radius vector to the intrinsic orientation of the nucleus.