Abstract
The zero-field splitting parameters D and E have been calculated using configuration interaction wave functions which minimize the triplet state energy and which take overlap realistically into account. The agreement with experiment is satisfactory in the case of the intra ring approximation (IRM), and significantly better than in the tight-binding approximation (TBM) for naphthalone, anthracene, phenanthrene, pyrene and 1,12-benzperylene.