Exact Shell-Model Calculation ofNi58andNi60

Exact configuration-mixing calculations have been done for ${\mathrm{Ni}}^{58}$ and ${\mathrm{Ni}}^{60}$, which have two and four neutrons, respectively, in unfilled shell-model orbitals. Several different types of effective potentials have been used; they are: (1) the surface-delta interaction, (2) Serber exchange, (3) a potential similar to the Rosenfeld mixture, (4) an approximate reaction matrix calculated with the Hamada-Johnston two-nucleon potential, and (5) Tabakin's nonlocal separable two-nucleon potential. These results are compared with those obtained by using the two-body matrix elements given by Cohen et al. (EIC) and by Auerbach (EIA). The overlap of the BCS projected wave function with the exact shell-model ground-state wave function is calculated. The odd-even mass difference obtained from pairing theory using these potentials is compared with the experimental values. It is found that the first three potentials give nearly the same quantitative results as that calculated with EIC and EIA, while the Hamada-Johnston and Tabakin potentials are rather weak.