  We calculate the single particle spectrum of the double magic nucleus
$^{78}$Ni in a Hartree-Fock approach using the Skyrme density-dependent
effective interaction containing central, spin-orbit and tensor parts. We show
that the tensor part has an important effect on the spin-orbit splitting of the
proton $1f$ orbit which may explain the survival of magicity so far from the
stability valley. We confirm the inversion of the $1f5/2$ and $2p3/2$ levels at
the neutron number 48 in the Ni isotopic chain expected from previous Monte
Carlo shell model calculations and supported by experimental observation.
