The yrast states in even-even $N=80$ nuclei are studied within the frameworks of both the large-scale shell model and the nucleon-pair approximation by using optimized effective interaction. Both calculations reproduce most of the experimental data for yrast $2_1^{+}\text{−}{8}_1^{+}$ states. Abrupt inconsistency between these two sets of calculations for the ${10}_1^{+}\text{−}{16}_1^{+}$ states is found to be a reflection of the type-II shell evolution, which is also seen in the effective single-particle energies of proton levels, at the ${10}_1^{+}$ states. This suggests a strong correlation between the type-II shell evolution and the evolution of nucleon-pair structure, which leads to fragility of optimized nucleon pairs of the ground states for states with spins higher than ten. By further consideration of nucleon pairs specifically optimized for the ${10}_1^{+}$ state with a new variational procedure in our configuration space, we are able to improve substantially the consistency between calculated results in the nucleon-pair approximation and those of the exact shell-model calculation.

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• Nuclear forces
• Nuclear structure & decays
• Shell model
• 90 ≤ A ≤ 149
• Nuclear many-body theory