Ground-state properties of La2NiO4 and the isostructural compound La2CuO4,
the parent material of some high-T-c superconductors, have been calculated
using the Hartree-Fock approximation (HFA). To our knowledge, this is the f
irst report in the literature in which calculations for these two materials
are done for an infinite crystal using the HFA. The results show that both
the nickelate and cuprate are antiferromagnetic (AFM) insulators, in agree
ment with experiments. The character of the highest occupied band in the cu
prate is found to be in-plane O 2p(x,y) strongly mixed with Cu 3d(x2).(y2),
agreeing with the hypotheses of most Hubbard models for this problem. The
spin densities show rather localized peaks with approximate cubic symmetry
at Ni sites (due to two singly occupied e(g) orbitals) or approximate fourf
old symmetry at Cu sites (due to d(x2).(y2)), and are small elsewhere. The
corresponding form factor agrees rather closely with our earlier cluster ca
lculations for the nickelate, while differing appreciably in the cuprate. W
e speculate on the reason for this. The results for the cuprate are consist
ent with magnetic neutron-scattering experiments: The shape of the form fac
tor is in overall qualitative agreement with that measured on a sample of q
uestionable stoichiometry; for a sample with presumably good stoichiometry,
on which only one Bragg peak was measured, the absolute intensity is in re
markably good agreement with our calculated result. The latter includes the
well-known correction for zero-point or quantum spin fluctuations. However
, the shape of the form factor for the nickelate is in serious disagreement
with experiment. We also calculated the energy splitting between AFM and f
erromagnetic states, and, for both materials, found the corresponding Heise
nberg exchange parameter J to be of the correct order of magnitude (about a
factor of 3 smaller than the experimental values). The calculated J value
for the cuprate is close to the result of a recent cluster Hartree-Fock cal
culation. We discuss the determination of J in density-functional theories,
as well as in the HFA, in the Appendix. [S0163-1829(99)02115-3].