Abstract Using effective interactions derived from a nuclear-matter G-matrix in local-density approximation we calculate natural-parity vibrations in 16O and 40Ca. The conventional RPA theory is extended to include 1p1h as well as 2p2h excitations in a consistent way. The extended theory introduces additional correlations which screen the long-range part of the effective interaction similar to Thomas-Fermi screening in the Coulomb gas. In this way the stability of the ground state against density fluctuations is enhanced. We find that the central part of the effective interaction plays the dominant role in coupling the isovector collective modes to high-energy 2p2h excitations.