The 2DES spectra in Fig. , Fig. , and Fig. are calculated with 10 4 trajectories, although convergence begins to appear with only 10 3 trajectories. The color bar corresponds to 2DES signal intensities normalized with respect to the maximum peak intensity for the 2DES at t 2 = 0 fs for each simulation. The monomer 2DES calculation involves T 1 = T 3 = 300 fs with a nuclear time step of 1 fs and 120 electronic time steps for each nuclear time step. For the 2DES intensity plot in Fig. we used 10 3 trajectories. This calculation involved T 1 = T 3 = 50 fs with a nuclear time step of 0.5 fs and 60 electronic steps per nuclear time step. In Fig. , all peak intensities are calculated by selecting a small region ∆ω = ±10 cm -1 around the off-diagonal peak location and integrating the intensity of the region, then normalized with respect to the integrated value at t 2 = 0 fs.