The Electronic Structure of La sub 2 CuO sub 4: Renormalization from Density Functional Theory to Strong Coupling Models

An internally consistent set of parameters for the extended, three band Hubbard model of the Cu-O sheets has been calculated from first principles using a constrained density functional approach. The Hubbard model with these parameters yields a super-exchange parameter (based on cluster calculations) in good agreement with experiment. The region of parameter space is such that the Cu-O based materials are intermediate between the extremes of charge fluctuation and spin fluctuation regimes.