For DKP, the 2D IR spectrum shows only one negative peak corresponding to ωa on the diagonal, together with an anharmonically shifted positive peak. The calculated anharmonic shift amounts to ∆ a,2a = 4 cm -1 . For NAGMA, the calculated 2D IR spectrum shows the two fundamentals on the diagonal, along with the corresponding overtones, with an anharmonic shift of ∆ a,2a = 7 cm -1 for the more intense antisymmetric mode at lower wavenumbers, and of ∆ s,2s = 4 cm -1 for the less intense symmetric mode. On the off-diagonal, peaks corresponding to the combination appear, with an anharmonic shift of ∆ as,a/s = 2 cm -1 .