Abstract
An exact method (full configuration interaction) and approximate schemes for the calculation of the anisotropy spin coupling matrix and the corresponding zero field splitting D parameter are considered. Calculation algorithms for the exact method and a special case (valence bond (VB) method) are also presented. The results of specific π calculations give evidence that the VB method in fact cannot be recommended for the description of the spin-spin interaction, especially for biradicals with the triplet ground state. At the same time, the Tamm-Dancoff method (taking into account all singly excited configurations) provides on average a good approximation to an accurate description of non-degenerate triplet states. The description of fine spin effects, which is rougher, but better than that in VB, is given by the restricted Hartree-Fock method for open shells.
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