2 J. Energies of the two doublet states were averaged. a zero trace, as in the case of Hubbard's Hamiltonian. Although the diagonal elements are small, they deviate from zero by 0.9-2.6 cm -1 . This artifact is a violation of time-reversal symmetry, which can be explained by a small spin contamination of EOM-SF-CCSD states entering the effective Hamiltonian construction. The effective exchange coupling constants J (shown in Table ) were extracted from the off-diagonal elements of the effective Hamiltonians and through the Landé interval rule . Bloch's and des Cloizeaux's forms of effective Hamiltonians predict nearly the same values of J, which is slightly different from the Landé rule. This can be attributed to small non-zero diagonal terms contributing to eigenvalues and entering the energy gaps used in Landé rule.