  Shell structures in the N\simeq Z nucleus ^{17}O and the neutron-rich oxygen
isotopes ^{23}O and ^{25}O are microscopically described by calculating
single-particle energies with modern nucleon-nucleon interactions within the
framework of the unitary-model-operator approach. It is found that the effect
of three-body cluster terms on the single-particle energy is more important in
^{23}O and ^{25}O than ^{17}O.

Nuclear Theory