The anharmonic vibrational excitation energies ωn and transition dipole moments µ nm calculated either quantum-chemically or with the help of a vibrational exciton model can be used as input for the calculation of 2D IR spectra. Here, we will only consider the static case, i.e., the 2D IR spectra are calculated for a fixed molecular structure. For additional theoretical background as well as the discussion of the dynamical case, we refer to Refs. 33, 91, 92. 2D IR spectroscopy probes the third-order nonlinear response. In general, three IR laser pulses are used, which lead to different Feynman pathways (see Fig. ), rephasing and non-rephasing, depending on the wave vector k = ∓k 1 ± k 2 + k 3 . The third-order response functions R n (t 1 , t 2 , t 3 ) are thus described by six double-sided Feynman diagrams that generate 2D IR spectra for any number of coupled eigenstates. The rephasing and non-rephasing response functions for any given number of eigenstates are calculated as