Heisenberg exchange in La 2 CuO 4

We study the exchange interaction in the quasi‐2D antiferromagnetic high T c superconductor parent compound La 2 CuO 4 by all‐electron, embedded cluster methods. Our material model is the cluster Cu 2 O 11 , embedded in a matrix of point charges. The SCF ground state configuration is characterized by Cu 2+ and O 2− with the d‐hole oriented in the [CuO 2 ] 2− plane along the CuO bonds. We admix to this an excited SCF configuration that has one Cu‐hole transferred to the central O into the ground state configuration by nonorthogonal CI , while paying special attention to the variational balance of the singlet and triplet states. This results in a value for the exchange parameter of J = −120 meV, in good agreement with the experimental value of J = −128 meV. © 1995 John Wiley & Sons, Inc.