Theoretical calculations of effective exchange integrals by spin projected and unprojected broken-symmetry methods. I. Cluster models of K_2NiF_4-type solids

摘要

Previously, various symetry-adapted (SA) and broken-symmetry (BS) computations have been performed for stronlgy correlated transition metal species so as to examine magnetic properties in simple cluster models such as binuclear transition metal oxides. Though SA computations such as simple cluster models such as binuclear transition metal oxides. Though SA computations such as the complete active space configuration interaction and CASSCF are desirable for estimating physical constant, these computations are heavy for larger cluster models wtih strongly correlated electrons. K_2NiF_4 is known to be the two-dimensional pervskite-type antiferromagnet and to have the electronic configuration similar to that of La_2CuO_4. Here, we have examined the utility and applicability of the BS spin-polarized hybrid-density functional theory (HUDFT) for cluster models of K_2NiF_4. As the result, HUDFT calculation such as UB2LYP has provided the reasonable effective exchange integral (J_(ab)) followed by our approximately spin projected scheme in comparison to the experimental one. It was also found that the squre planar tetranuclear models has provided the most reasonable J_(ab) value by HUDFT. In addition, external effects such as putting point charges around cluster models and changing distances between nickel and fluorine have been also examined. The natural orbital analysis by HUDFT has been carried out to obtain natural orbitals and their occupation numbers. Charge density, spin density, and chemical indices expressed by the occupation numbers have been also obtained to elucidated the nature of the chemical bonds in the K_2NiF_4-type solids.