Figure shows the calculated 2D IR spectra for the two considered polarization condi- A comparison of the 2D IR spectra calculated using L-VSCF/L-VCISD with full anharmonic two-mode potentials (see Fig. and) and with harmonically approximated two-mode potentials (see Fig. and) shows only minor differences, in particular for the Fermi resonance of the overtone of the axial CO stretch vibrations. This confirms that the computationally efficient approximation of using L-VSCF/L-VCI with anharmonic one-mode potentials and harmonic two-mode potentials (1-mode+2h) is applicable for the CO stretch vibrations in carbonyl complexes. Based on this approximation, it is possible to set up the L-VSCF exciton model as described above. The frequency parameters used in this model are listed in Table . The resulting 2D IR spectra (see Fig. and) are in very good agreement with the L-VSCF/L-VCISD results, even though the peak positions are shifted by a few wavenumbers (see also Table ). These differences are due to the approximations inherent to the vibrational exciton model compared to a full L-VCISD treatment. The L-VSCF exciton model can be closely approximated by using the harmonic local-mode frequencies in combination with an anharmonic shift of ∆ = 21 cm -1 (see Table ). The corresponding 2D IR spectra for this ∆-exciton model (see Fig. and) are almost indistinguishable from those based on the L-VSCF exciton model.