The role of HOOP-modes in the ultrafast photo-isomerization of retinal models

Molecular dynamics calculations of three-double-bond retinal models have been performed in vacuo to study the effect of the surface hopping criterion on the quantum yield in the simulated cis-trans photoreaction. The study reveals that the product distribution changes dramatically, when surface hopping occurs only very close to surface crossings. The HOOP-coordinate of the isomerizing double bond is strongly involved in the generation of the photoproduct. The effect is documented by systematic variation of molecular parameters after the surface hop. An empirical rule deduced from the analysis simplifies the calculation of trajectory ensembles for retinal models and gives new insights into the mechanism of cis-trans photo-isomerization in protonated Schiff bases.