The Importance of Going beyond the Independent Atom Model When Predicting UED Signals from Simulations

The level of theory required to predict ultrafast electron diffraction signals is investigated in photoexcited γ-butyrolactone. The total isotropic diffraction signal is calculated both with the independent atom model (IAM) and directly from ab initio electronic wave functions. The results are benchmarked at the equilibrium geometry, along a representative photochemical reaction coordinate (for both ground and excited electronic states), and, most importantly, for a full fewest-switches surface-hopping simulation of the nonadiabatic dynamics. The IAM qualitatively captures the time-resolved UED signal, but there are deviations between the IAM and the ab initio results, irrespective of the electronic structure method used. The errors of the IAM do not get washed out even when the nuclear wavepacket is considered. The errors primarily manifest in the intensity rather than position of scattering features, particularly at small to medium values of momentum transfer, and are sufficient to affect lifetimes and other parameters determined from the diffraction signal. Our results indicate that the IAM can lead to errors when scattering signals are inverted. Crucially, calculations beyond the IAM are necessary to account for the effects of chemical bonding, charge transfer, electronic excitation and ionization.