SMALL-BANDWIDTH PERTURBATION-THEORY FOR HIGHLY COVALENT MOTT INSULATORS

Calculation of the elastic neutron scattering form factor by an essent ially standard approach has given results that disagree seriously with experiment an La2NiO4. This has motivated us to look for a more funda mental approach to such a calculation (in Mott insulators). We have be gun by considering perturbation approaches in the context of the three -band Hubbard model of cuprate CuO2 planes due to Hybertsen et al. Thi s was recently shown, for a small cluster, to have a straightforward s mall-bandwidth perturbation expansion of the Heisenberg exchange param eter J that is nonconvergent. We study the roles of one-body and two-b ody transformations on the basis set of states in converting nonconver gent many-body perturbation expansions into convergent ones. We choose the one-body transformations guided by the thermal single determinant approximation (TSDA), a variational generalization of the thermal Har tree-Fock approximation. All transformations preserve ''localization'' of copper d orbitals, and thus lead to low-lying states governed by a Heisenberg spin Hamiltonian, in leading order, provided the perturbat ion theory is convergent. We find the one-body transformations do make the perturbation expansion converge, although rather slowly; addition of two-body transformations gives significant improvement in the conv ergence rate. The reason for the limitation of the one-body transforma tion is given.