Comparing our results to the experimental spectra measured by Khalil et al. shows a good agreement with the overall line shapes and relative peak positions, but we find that our spectra are shifted to higher wavenumbers. For a more detailed comparison, For all calculations, the positions of the fundamentals are shifted compared to experiment, which is due to the shortcoming of density-functional theory as employed for the vibrational frequency calculations. However, our calculations provide a rather good agreement for the splitting between the two fundamentals, which is calculated as 61 cm -1 , while it amounts to 69 cm In addition, Table compares three different anharmonicities: the anharmonicity of the doubly-excited antisymmetric CO stretch mode ∆ 2a,a (i.e., the difference between the two peaks in the lower left corner of the 2D IR spectra), the anharmonicity of the doublyexcited symmetric CO stretch mode ∆ 2s,s (i.e., the difference between the two peaks in the upper right corner), and the anharmonicity of the the combination band ∆ as,a/s (i.e., the difference between the two peaks in the upper left and in the lower right corner). All calculations are in good agreement with the experimental anharmonicities, even though they underestimate them by a few wavenumbers.