The calculated 2D IR spectra of DAR are shown in Fig. . The fundamentals show up as negative peaks on the diagonal, which are each accompanied by a negative peak that is due to the excitation to the corresponding overtone. The coupling between the modes results in cross-peaks, where the anharmonically shifted cross-peaks correspond to the excitation of the combination band. Different polarization conditions vary the intensities of the cross-peaks, which are comparably weak in the ⟨ZZZZ⟩ polarization condition, but can be strongly enhanced by using the ⟨ZZXX⟩ polarization condition. In our calculations of the 2D IR spectrum of DAR, we compare different computational approximations. Our most sophisticated approach uses a two-mode approximation potentialenergy surface [see Eq. ( )] in combination with L-VSCF/L-VCISD calculations. The corresponding results are shown in Fig. and for the ⟨ZZZZ⟩ and ⟨ZZXX⟩ polarization conditions, respectively. These spectra serve as reference for the following, more approximate models. As a first simplification, we only calculate the anharmonic one-mode potentials and approximate the two-mode potential harmonically [see Eq. ( )]. This drastically cuts down the computational costs for the construction of the potential energy surface. The resulting 2D IR spectra are shown in Fig. and and are virtually indistinguishable from those obtained with the fully anharmonic two-mode potentials, which confirms that this approximation is valid.