CORRELATION-ENERGY IN SMALL JELLIUM CLUSTERS

The correlation energy of finite systems of electrons in a neutralizin g spherical positively charged background (jellium) is investigated wi thin various approximations. The correlation energy is defined with re spect to the Hartree-Fock energy (with exact treatment of exchange). O n one hand, the exact second-order energy contribution is calculated f rom many-body perturbation theory. On the other hand, a quasi-boson ap proximation to harmonic order only allows one to estimate the RPA grou nd-state correlation energy and to emphasize the relative importance o f Coulomb-interaction screening corrections (ring diagrams). One finds that the second-order contribution accounts for most of the correlati on energy. Comparisons with other approaches, such as density-function al theory and CI calculations, confirm this assertion.