This spin Hamiltonian will be referred to as 'model d2'. Additional parameters are the residual ZFS-tensor described by D 0 and the g-tensors for each Co site and state, g k i , and for the radical bridge g b . As in the previous 'model d1', eq. 1, the latter is assumed to be isotropic with a fixed value of g b = 2.0. The g k i are assumed diagonal with the z component aligned with the main magnetic axis of the overall system and the exchange coupling strength was assumed to be the same in ground and excited states of the individual ions. Only the z-components were included in the fit to the energy levels and to the µ z values from the ab initio computations. The parameters of both cobalt centers are the same, because the two centers are related by inversion symmetry. The fit results in ∆ = 712 cm -1 , γ = 344 cm -1 and J = 317 cm -1 (Table ). The splitting of the quartet states ∆ is somewhat larger than for the mononuclear species (∆ = 478 cm -1 ). This is mainly an effect of the different computational procedure in the case of 2, compared to 1: Fewer states were included in the determination of the averaged orbitals, which was necessary in view of the large size of the system. In a computation of the analogous species in which one cobalt ion has been replaced by zinc, using the same procedure as for the mononuclear species, we find a splitting of 540 cm -1 , which is much closer to what is found in the mononuclear case. This demonstrates the limits of the accuracy of our computational procedure, but does not invalidate the overall picture. A similar deviation is found for the D 0 parameter, which is negative in the 'model d2' fit (-14 cm -1 ) but positive for the mononuclear species (+15 cm -1 , see above).