Application of extended random-phase approximation with ground-state correlations to collective excitations of 16O

The ground-state correlation effects on the octupole excitation of 16O are studied using the extended random phase approximation (ERPA) derived from the time-dependent density-matrix theory. ERPA includes the effects of ground-state correlations through the fractional occupation of single-particle states and a correlated two-body density matrix. The coupling to two particle – two hole configurations is also included in ERPA. It is found that the first 3− state calculated in the random phase approximation (RPA) is shifted upward when the self-energy contributions due to ground-state correlations are included in particle – hole pairs and that the coupling to two particle – two hole states plays a role in shifting the first 3− state down to the right position.