Figure 4.

Energy level scheme for a system of two coupled oscillators. The isolated peptide states (Left) are coupled by some weak interaction, which mix them to generate the excitonic states (Right). Anharmonicity, which is crucial for understanding the 2D pump-probe spectra, is introduced into this model by lowering the energies of the double-excited monomeric site states |i〉|i〉 by Δ from their harmonic energies 2ɛ_ii. This anharmonicity mixes into all coupled states, giving rise to diagonal anharmonicity (Δɛ_kk) and off-diagonal anharmonicity (mixed-mode anharmonicity, Δɛ_kl) in the basis of the normal modes discussed in the text.