Simple isoenergetics of isovector giant multipole resonances via an isoinvariant RPA

Displacements of the energy centroids of the distinct isospin fragments of the isovector giant resonances of nuclear multipole excitations are obtained within the framework of an isoinvariant RPA. The isovector contribution of the displacement is shown to be independent of the ground-state correlations (GSC) as well as of the multipolarity, whereas the isotensor contribution is found to have a simple dependence on multipolarity and a vary vital dependence on the GSC. The two parameters measuring the above contributions have been estimated numerically using a few well established model and empirical parameters, and the results agree quite well with the existing experimental findings. These also include confirmation of multipolarity independence of the isovector contribution and the essentially renormalising effect of the isotensor contribution. (author)