CLUSTER STUDIES OF LA2CUO4 - A MAPPING ONTO THE PARISER-PARR-POPLE (PPP) MODEL

The techniques of ab initio electronic structure theory are used to study Cu2O7 and Cu2O11 cluster models of La2Cu04. Fair agreement is obtained with the experimentally determined spin exchange constant J (90 meV calculated vs 125 meV measured) at the expense of quite large configuration interactions (CI) expansions. Results for various charge states of the cluster are well described by a ''single-band'' Pariser-Parr-Pople (PPP) model. As in earlier local-density-functional (LDF) based parameter determinations, the present work suggests these materials fall in the strong coupling regime. However, a significant intersite Coulomb repulsion is found in the present research. It is of sufficient strength V approximately U/5 to indicate that charge fluctuations may be more important in these materials than generally believed.