Initial step of the photoprocess leading to vision only requires minimal atom displacements in the retinal molecule

The primary processes of vision and the light driven proton pump of some bacteria involve cis–trans or trans–cis isomerization, respectively, of a retinal molecule. The time constant of the initial process, 200 fs according to spectroscopic measurements, is very small and hard to explain as a twist around one double bond. In this Letter we have shown, using CASSCF calculations, that the important avoided crossing (AC) may be reached in an essentially down-hill reaction while the retinal chromophore remains in the protein cavity.