  Motivated by the recent experimental discovery of superconductivity in the
infinite-layer nickelate Nd0.8Sr0.2NiO2 [Li et al., Nature 572, 624 (2019)], we
study how the correlated Ni 3dx2-y2 electrons in the NiO2 layer interact with
the electrons in the Nd layer. We show that three orbitals are necessary to
represent the electronic structure around the Fermi level: Ni 3dx2-y2, Nd
5d3z2-r2, and a bonding orbital made from an interstitial s orbital in the Nd
layer and the Nd 5dxy orbital. By constructing a three-orbital model for these
states, we find that the hybridization between the Ni 3dx2-y2 state and the
states in the Nd layer is tiny. We also find that the metallic screening by the
Nd layer is not so effective in that it reduces the Hubbard U between the Ni
3dx2-y2 electrons just by 10--20 %. On the other hand, the electron-phonon
coupling is not strong enough to mediate superconductivity of Tc ~ 10 K. These
results indicate that NdNiO2 hosts an almost isolated correlated 3dx2-y2
orbital system. We further study the possibility of realizing a more ideal
single-orbital system in the Mott-Hubbard regime. We find that the Fermi
pockets formed by the Nd-layer states dramatically shrink when the
hybridization between the interstitial s state and Nd 5dxy state becomes small.
By an extensive materials search, we find that the Fermi pockets almost
disappear in NaNd2NiO4 and NaCa2NiO3.
