FLUCTUATION-EXCHANGE THEORY FOR GENERAL LATTICE HAMILTONIANS

The fluctuation-exchange or FLEX, approximation for interacting electr ons is derived for lattice Hamiltonians with general instantaneous one - and two-body terms. The use of a two-body basis set indexed by relat ive separation, rather than relative momentum, is emphasized. The fluc tuation-exchange approximation for the three-orbital CuO2 model with o n-site and near-neighbor Coulomb interactions is solved for one-partic le properties. Unit-cell densities corresponding to both ''hole doping '' and ''electron doping'' are studied. The model is found to be far f rom a charge-density instability for all reasonable parameter values. The only nearly unstable particle-hole channel for unit-cell densities close to unity has Q similar to(pi,pi) and S=1 (antiferromagnetic). T he Fermi surface of the interacting system is computed, and the Luttin ger theorem verified numerically in its most general context. Orbital- projected occupancy factors and spectral densities an examined.