For comparison, Figure includes an experimental ∆PDF (red), obtained at a pump-probe delay of 550 fs, i.e., long after the photoexcited population has relaxed from the excited state according to previous spectroscopic studies (see above). The ∆PDF can be in general compared to two types of simulated signatures, generated either by subtracting simulated PDF signatures (e.g., within IAM) of a single reactant equilibrium geometry from the one of a single product geometry (product ∆PDFs) or by subtracting the averaged PDF signatures of an ensemble of starting geometries from those of an ensemble of geometries taken from a specific time point of a nonadiabatic wavepacket simulation (transient ∆PDFs). The ∆PDF from Fig. qualitatively fits to product ∆PDFs by showing negative signatures at 1.4 Å and 2.4 Å and positive signatures above 3 Å. However, the shape of the positive signature does not match well with either of the product ∆PDFs. This is not surprising, since the available vibrational energy in the photoproducts is substantially higher than the potential barriers between the different hexatriene isomers. A quantitative agreement can be achieved by transient ∆PDF signatures from explicit wavepacket simulations of the nonadiabatic dynamics reaching all the way to the ground state of the photoproduct, as shown in Figure . The nonadiabatic dynamics simulations were performed using starting molecular geometries sampled from a Wigner distribution in the ground state minimum of CHD, which were further filtered according to their ππ * excitation energies to better match the spectrum of the excitation pulse in the experiment . These starting geometries were propagated in the excited state potential using ab initio multiple spawning (AIMS) with α-SA2-CASSCF(6,4)/6-31G * for the electronic structure . AIMS trajectory basis functions that returned through the CI to the electronic ground state were further propagated using uPBE0/6-31G * . The transient ∆PDF signatures in Fig. & 4d were generated by first simulating IAM diffraction based on the dynamics simulations and transforming them, truncated to the same s-range, into real space using the same code as for the experimental data. The lower limit (1.3 Å -1 ) of the experimental s-range (range of scattering angles) originates from a hole in the detector through which the undiffracted electrons are transmitted. The upper limit of 10 Å -1 in the CHD data results from the strong sdependence of electron scattering cross-section and the associated deterioration of the diffraction signal at large s values. The ∆PDF(s, t) in Fig. & 4b demonstrate quantitative agreement between experiment and simulation. They also show clear signatures from the initial ring-opening motion. The increase of the negative signature at 2.4 Å around time zero shows a 70 fs delay with respect to the negative signature at 1.4 Å, which is visible by the time-dependent change in their relative intensity in Fig. & 4b . This effect can be easily understood with the help of the above-described relationship of ∆PDF and distance change. The increase in the value of R1 during the ring-opening is accompanied by an increase in the R2 values leading to negative contributions at 1.4 Å and 2.4 Å and positive contributions at distances > 1.4 Å and > 2.4 Å. As long as R1 exhibits values in the range of 2.4 Å, the positive contribution associated with R1 partially compensates for the negative contribution associated with R2, which leads to the observed delay in the increase of the negative signature at 2.4 Å. Thus, the delay represents the timescale of nuclear wavepacket motion leading to the expansion of the C-C bond from 1.4 Å to 2.4 Å. The onset of the positive signatures in the ∆PDFs > 3 Å is co-timed with the negative signature at 2.4 Å and does not show a measurable distance dependence. This observation can only be interpreted as a significant acceleration in the ring-opening motion when R1 passes a distance of approximately 2.4 Å. The acceleration is expected, since the CI with the ground state is reached at an R1 distance of about 2.2 Å. The potential energy of the CI is much closer to the energy of the Franck-Condon region in S1 than to the energies of the photoproduct minima (see Fig. ). Therefore, the majority of the photoabsorbed energy is released into the ring-opening nuclear degree of freedom directly after internal conversion to the ground state by the acceleration of the nuclear wavepacket along a steep gradient towards the photoproduct minima. This degree of freedom maps well enough onto the interconversion modes between the different photoproduct isomers (see Fig. ) and is simultaneously decoupled enough from the other vibrational modes in the molecule that the ringopened population can be observed to coherently oscillate between structures resembling the photoproduct isomer equilibrium geometries for the first 800 fs after photoexcitation (see the red center-of-mass lines in Fig. &).