The role of the 4p-4h excitations in the microscopic description of56Ni

Abstract

An effective Hamiltonian for shell model calculations in the middle of the 2p-1f shell is microscopically deduced from the Tabakin nucleon-nucleon potential. Its matrix elements are tabulated. Such a Hamiltonian is applied to the description of56Ni. All configurations up to four particle-hole excitations are considered and a weak-coupling assumption is used to select the relevant shell model states. The weakcoupling assumption turns out to be very effective in reducing the dimensions of the secular matrices. No phenomenological parameter is used. Only a small adjustment of the 1f2/7-2p2/3 gap with respect to its theoretical value is allowed. The adopted value is 2.65 MeV. The inclusion of the 4p-4h configurations is essential to get a reasonable agreement between the theoretical and experimental spectra of56Ni. The ground state turns out to be 61% closed and 36% 2p-2h, while the excited states are almost pure 4p-4h.