  Motivated by recent experiments, we use the $+U$ extension of the generalized
gradient approximation to density functional theory to study superlattices
composed of alternating layers of LaNiO$_3$ and LaMnO$_3$. For comparison we
also study a rocksalt ((111) double perovskite) structure and bulk LaNiO$_3$
and LaMnO$_3$. A Wannier function analysis indicates that band parameters are
transferable from bulk to superlattice situations with the exception of the
transition metal d-level energy, which has a contribution from the change in
d-shell occupancy. The charge transfer from Mn to Ni is found to be moderate in
the superlattice, indicating metallic behavior, in contrast to the insulating
behavior found in recent experiments, while the rocksalt structure is found to
be insulating with a large Mn-Ni charge transfer. We suggest a high density of
cation antisite defects may account for the insulating behavior experimentally
observed in short-period superlattices.
