The simplest interpretation for the resonance at 27.5 cm À1 in an S = 1 system would be axial ZFS with D = 27.5 cm À1 and vanishing E. However , simulations with these parameters did not reproduce the field dependence of the FD - FT THz - EPR spectra ( Supporting Information , Figure S5 ) . The alternative scenario , where D is negative and the rhombicity in the ZFS lifts the degeneracy of the m s = AE 1 levels is well in accordance with the experimental data . This assumption predicts two EPR transitions at j D j+j E j and 2 j E j . The observed transition at 27.5 cm À1 then corresponds to 2 j E j % 27.5 cm À1 . This yields j E j % 13.5 cm À1 and ( due to j E j j D j /3 ) creates a constraint on D < À41 cm À1 . The second EPR transition is then expected at j D j+j E j , which would be above 54 cm À1 and therefore outside the spectral window of the data in Figure 9 .