Multiparticle-multihole states around 16O and correlation-energy effect on the shell gap

Multiparticle-multihole states around O-16 such as the famous 0(2)(+) state in O-16 are investigated with a shell-model calculation in the full p-sd model space. It is found that the modification of the N = Z = 8 shell gap due to correlation energy is unexpectedly large, which plays an essential role in accounting for those states. We point out that correlation energy affects the shell gap in the opposite way between O-16 and Ni-56, giving its mechanism in terms of the difference of the L-S and j-j closures.