Quantum Dynamics Predicts Coherent Oscillations in the Early Times of a Biological Photoisomerization

In this work, we study the quantum dynamics of the isomerization associated with the primary event in vision employing a model for the 2-cis-penta-2,4-dieniminium cation (cis-PSB3). We aim to demonstrate that the observed relationship between a specific wag mode and the reaction quantum yield emerges naturally from wavepacket propagation. To do so, we address two previously undetected methodological issues related to (i) establishing the appropriate level of convergence of the quantum dynamics calculations, and (ii) describing the emergence of distinct oscillatory behaviors during the formation of the cis and trans isomers in the ground state following cis-PSB3 photoexcitation. The two issues are strictly related, since only upon reliable convergence, the simulated dynamics is able to capture the large amplitude motion associated with the torsional and wag deformations in the region of the reactive bond.