  The eigensolutions of the collective Hamiltonian with different potentials
suggested for description of the isovector pair correlations are obtained,
analyzed and compared with the experimental energies. It is shown that the
isovector pair correlations in nuclei around $^{56}$Ni can be described as
anharmonic pairing vibrations. The results obtained indicate the presence of
the $\alpha$-particle type correlations in these nuclei and the existence of
the interaction different from isovector pairing which also influences on the
isospin dependence of the energies.
