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The Ni ion in LaNiO$_2$ has the same formal ionic configuration $3d^9$ asdoes Cu in isostructural CaCuO$_2$, but it is reported to be nonmagnetic andprobably metallic whereas CaCuO$_2$ is a magnetic insulator. From ab initiocalculations we trace its individualistic behavior to (1) reduced $3d-2p$mixing due to an increase of the separation of site energies ($\epsilon_d -\epsilon_p$) of at least 2 eV, and (2) important Ni $3d(3z^2-r^2)$ mixing withLa $5d(3z^2-r^2)$ states that leads to Fermi surface pockets of La $5d$character that hole-dope the Ni 3d band.Correlation effects do not appear to belarge in LaNiO$_2$. However, ad hoc increase of the intraatomic repulsion onthe Ni site (using the LDA+U method) is found to lead to a novel correlatedstate: (i) the transition metal $d(x^2-y^2)$ and $d(3z^2-r^2)$ states undergoconsecutive Mott transitions, (ii) their moments are antialigned leading(ideally) to a "singlet" ion in which there are two polarized orbitals, and(iii) mixing of the upper Hubbard $3d(3z^2-r^2)$ band with the La $5d(xy)$states leaves considerable transition metal 3d character in a band pinned tothe Fermi level. The magnetic configuration is more indicative of a Ni$^{2+}$ion in this limit, although the actual charge changes little with U.Comment: 7 pages, 8 figure

physical review. b, condensed matter and materials physics

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Infinite-layerLaNiO2: Ni1+is not