Accurate ab initio determination of magnetic interactions and hopping integrals in La2-xSrxCuO4 systems

The nature of magnetic interactions and electron transfer processes in La2-xSrxCuO4 systems are studied, by means of an ab initio embedded cluster model approach, using a difference dedicated configuration interaction (DDCI) procedure. For the undoped system, the crucial role played by the ligand to metal charge transfer (LMCT) configurations in the magnetic process makes necessary the use of an enlarged DDCI space, which explicitly takes account of the relaxation of these LMCT configurations. This procedure allows us to approach the experimental magnetic coupling constant value. In hole-doped systems, the value obtained for the electron transfer integral, t, is of 0.54-0.57 eV. The extra hole, characterized from the nature and occupation of differential natural orbitals, has a strong p character (approximately 50%) and is essentially localized in CuO2 planes. These results are in agreement with the experimental evidence about these kinds of compounds. Neither the value of t nor the nature of the extra hole are seriously affected by the optimization of the orbitals used in the CI expansion. This suggests that a t-J effective Hamiltonian is an adequate model to study the electronic properties of these systems. (C) 2000 American Institute of Physics. [S0021-9606(00)30210-0].