Realistic electronic structure calculations for magnetic insulators like La2CuO4.

We suggest that Hartree-Fock theory is a good starting point for the solution of {ital ab} {ital initio}--derived extended Hubbard Hamiltonians for magnetic insulators like La{sub 2}CuO{sub 4}, as indicated by surprisingly accurate results for the superchange frequencies and insulating gaps for this material and isostructural La{sub 2}NiO{sub 4}, K{sub 2}CuF{sub 4}, and K{sub 2}NiF{sub 4}. Limited configuration interaction beyond Hartree-Fock theory is then used to demonstrate that admixtures of {ital a}{sub 1}- ({ital d}{sub 3{ital z}/{sup 2}{minus}{ital r}{sup 2}} -) symmetry states at energies somewhat away from the gap edge are essential to the dispersion of the first quasiparticle branch in La{sub 2}CuO{sub 4} for doped-in holes.