Abstract: The S₁→S₀ nonadiabatic coupling terms are calculated for different reaction modes of concerted [2+4] photocycloadditions in the Huckel-Hubbard approximation.The nonadiabatic transition probabilities are estimated by means of the Landau-Zener model as well.The calculated nonadiabatic coupling terms and transition probabilities show clearly the differences between the nonadiabatic interactions of the two reaction modes s+a (or a+s) and s+s.With these calculated results the stereospecifities in concerted [2+4] photocycloadditions are correctly predicted.

Key words: Photocycloaddition, Nonadiabatic coupling, Transition probability.