INSTABILITY OF THE HF GROUND-STATE IN NI-56 CAUSED BY ALPHA-LIKE 4-NUCLEON CORRELATIONS

The problem of whether the doubly-closed-shell state of Ni-56 is Stabl e or not against the 4p-4h excitations caused by the a-like four-nucle on correlations is investigated by testing the stability condition of the HF ground state. First, the stability condition of the HF ground s tate against the alpha-like four-nucleon correlations is reformulated. Second, the stability of the HF ground state in Ni-56 is examined usi ng the refined stability condition. The calculations with the use of a vailable effective interactions indicate that the HF ground state with the double magic number Z=N=28 is unstable against the alpha-like 4p- 4h excitations.