  We investigate the ferrimagnetism and ferroelectricity of bulk NiFe$_2$O$_4$
with tetragonal $P4_122$ ~symmetry by means of density functional calculations
using generalized gradient approximation + Hubbard $U$ approach. Special
attention is paid to finding the most energetically favorable configuration on
magnetic ordering and further calculating the reliable spontaneous electric
polarization. With the fully optimized crystalline structure of the most stable
configuration, the spontaneous polarization is obtained to be 23 $\mu$C/cm$^2$
along the z direction, which originates from the hybridization between the 3d
states of the Fe$^{3+}$ cation and the 2p states of oxygen induced by
Jahn-Teller effect.
