  The single-particle spectral function of 56Ni has been computed within the
framework of self-consistent Green's functions theory. The Faddeev random phase
approximation method and the G-matrix technique are used to account for the
effects of long- and short-range physics on the spectral distribution.
  Large scale calculations have been performed in spaces including up to ten
oscillator shells. The chiral N3LO interaction is used together with a monopole
correction that accounts for eventual missing three-nucleon forces. The
single-particle energies associated with nucleon transfer to valence 1p0f
orbits are found to be almost converged with respect to both the size of the
model space and the oscillator frequency.
  The results support that 56Ni is a good doubly magic nucleus. The absolute
spectroscopic factors to the valence states on A=55,57 are also obtained. For
the transition between the ground states of 57Ni and 56Ni, the calculations
nicely agree with heavy-ion knockout experiments.
