where the index c runs over the two cobalt centers. Due to the inversion symmetry of the system, the values of the derivatives at the two centers have either even or odd parity, depending on the parity of the mode indexed by a. Theoretical considerations suggest that the parity of the ZFS derivatives and that of the vibrational modes are the same, which is also confirmed by numerical results, see supporting information section S4.5. These numerical studies also show that the D order (see above). Like for the FIR transitions at low energy, we have to assume that non-linear couplings and anharmonic effects in the phonon spectrum lead to this signal. A third Raman line is observed at 917 cm -1 . This band is clearly much broader than the Raman bands observed at lower energies and the rather smooth magnetic-field-dependence suggests that this band corresponds to an electronic transition to a state of the same multiplicity as the ground state, analogously to the highest energy band discussed for 1. In fact, the ab initio computations show that further sextet states are expected in this energy region and so do the spin Hamiltonian models. From the simpler 'model d1', we obtain a single spin ladder which only predicts two sextets, the ground state and the excited one at ≈ 3J + |D|. With the expected values for J and D this is clearly much higher than the observed excitation at 917 cm -1 . The 'model d2' (as well as the the ab initio computation) suggests additional spin ladders due to the low-lying electronic excited states at the Co centers. Transitions to these states are the more likely candidates for the assignment of the 917 cm -1 peak. In fact, two near degenerate states are expected for the second spin ladder (see also supporting information section S3.4), but they must have different parity and the transition to only one of them is Raman active. Simulations according to the 'd2 model' imply that this assignment translates into a value of ∆ ≈ 420 cm -1 for the energy gap between the two single-ion quartet states, which is smaller than what is found for the mononuclear compound 1 (∆ = 500 cm -1 ). The ab initio computations do not suggest such a reduction of ∆ and, therefore, predict all the sextet states at higher energies. This may be attributed to the limits of the accuracy of these computations, in particular as we have neglected spin-orbit coupling to further, higher-lying states.