The ground state wave functions of 12C, 16O, 40Ca and 208Pb are calculated within the framework of the random phase approximation to all orders in the number of excited particle-hole pairs. Large amounts of shell breaking are found; the probability for the double closed shell configuration is thus only 0.436 in 12C, 0.665 in 16O, 0.331, in 40Ca and 0.555 in 208Pb. Occupation numbers are small in 208Pb (≦0.08), moderate in 16O and 40Ca (≈ 0.1) and large in 12C (≦0.26). An exactly soluble model shows that the RPA solution for such large shell breaking still yields fairly accurate energies but underestimates the shell breaking. © 1969.