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Abstract

The LDA+U method is used to calculate exchange integrals in strongly correlated cuprate compounds. We distinguish two approaches. The first one compares directly the total energies of different collinear spin arrangements with the corresponding ones of Heisenberg-like models. The second approach maps the energy of noncollinear spin-spiral states to the mean-field solutions of the effective spin Hamiltonian. Both approaches are applied to Sr2CuO2Cl2 which can be described with good accuracy by a two-dimensional Heisenberg model with only nearest-neighbor exchange. It is shown that the consideration of quantum fluctuations improves the resulting exchange integrals. The variation of the results with U and the difference between the two approaches are small. Both methods have also been applied to Ba2Cu3O4Cl2 which has two coupled antiferromagnetic spin systems. The coupling between the two subsystems has been shown to be larger than previously estimated.