Structure of the random-phase-approximation ground-state wave functions of Ni56 and Ca48

Conventional random-phase-approximation (RPA) and corrected RPA (MRPA) calculations are carried out for Ni56 and Ca48 nuclei, using the Tabakin potential without and with secondorder Born corrections. The obtained ground-state wave functions are analyzed. The probability for the doubly-closed-shell (zero-particle, zero-hole) configurations is found to be small for Ni56 [18% (RPA), 38% (MRPA)]. Similar numbers for the Ca48 nucleus are, 35% RPA, 56% (80% for protons and 69% for neutrons) MRPA. In the case of Ni56, the T=0 part contributes 95% to the shell breaking. The occupation numbers for single-particle and single-hole states are small except for the 1f72 hole and 1f52 particle states in Ni56, and for the 1d32 proton-hole state in Ca48. It is found that the conventional RPA using antisymmetrized matrix elements overestimates the calculated quantities of the ground state, roughly by a factor of 2. The effect of truncating the configuration space is also studied quantitatively. The results are compared with earlier investigations.