  We perform ab initio calculations for nuclei around ^{16}O, ^{40}Ca, and
^{56}Ni using realistic nucleon-nucleon forces. In particular, ^{56}Ni is
computed as the heaviest nucleus in this kind of ab initio calculation.
Ground-state and single-particle energies including three-body-cluster effects
are obtained within the framework of the unitary-model-operator approach. It is
shown that the CD-Bonn nucleon-nucleon potential gives quite good results close
to the experimental values for all nuclei in the present work.
