Realistic Electronic Structure Calculations for Magnetic Insulators Like La₂CuO₄

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 La₂CuO₄, as indicated by surprisingly accurate results for the superexchange frequencies and insulating gaps for this material and isostructural La₂NiO₄, K₂CuF₄, and K₂NiF₄. Limited configuration interaction beyond Hartree-Fock theory is then used to demonstrate that admixtures of a₁ (d_3z²-r²) symmetry states at energies somewhat away from the gap edge are essential to the dispersion of the first quasiparticle branch in La₂CuO₄ for doped-in holes.