Cluster studies of La2CuO4: A mapping onto the Pariser–Parr–Pople (PPP) model

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Abstract
The techniques of ab initioelectronic structure theory are used to study Cu2O7 and Cu2O11 cluster models of La2CuO4. 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∼U/5 to indicate that charge fluctuations may be more important in these materials than generally believed.