Theory of elementary excitations in closed shell nuclei

A theoretical approach is discussed which allows the calculation of both ground and excited state properties of nuclei in the same framework. The energy of the nucleus, calculated with effective density dependent interactions, is considered as a functional of the one-body density matrix. Starting from this functional, the Hartree-Fock equations for the ground state and the random phase approximation for the excited states are derived. Thus the same effective nucleon-nucleon interaction is used for the ground state and for the excited states: this yields self-consistent effects which are shown to be very important for the properties of the collective states. This approach may be viewed as a microscopic basis for the unified model. A method is developed to solve the RPA equations in the self-consistent basis. It allows the use of any phenomenological form of interaction, in particular finite range ones. (Auth.)