Realistic Electronic Structure Calculations for Magnetic Insulators Like La_{2}CuO_{4}

Abstract

We suggest that Hartree-Fock theory is a good starting point for the solution of ab initio-derived extended Hubbard Hamiltonians for magnetic insulators like ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$, as indicated by surprisingly accurate results for the superexchange frequencies and insulating gaps for this material and isostructural ${\mathrm{La}}_{2}$Ni${\mathrm{O}}_{4}$, ${\mathrm{K}}_{2}$Cu${\mathrm{F}}_{4}$, and ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$. Limited configuration interaction beyond Hartree-Fock theory is then used to demonstrate that admixtures of ${a}_{1} ({d}_{3{z}^{2}\ensuremath{-}{r}^{2}})$ symmetry states at energies somewhat away from the gap edge are essential to the dispersion of the first quasiparticle branch in ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ for doped-in holes.