The second, indirect path calculated for the PPMet-4 complex adds new insights, concerning the immediate N-O interactions in the PPMet-4 Franck-Condon region. As indicated by the bottomleft panel of Figure , a weak attraction between the MeOH oxygen and the nitrogen atom of the pyridine ring drives the system in a barrierless manner towards a potential energy surface (PES) region of rather small S 1 /S 0 energy difference (¡ 1.0 eV). From this conformation, if one allows gradual OH distance relaxation (bottom-righ panel of Figure ), the S 1 /S 0 degeneracy advances even further, almost immediately leading to an energy crossing, as the proton is transferred from MeOH to PrdPyr. The topography of the indirect path is in full agreement with the result of unconstrained S 1 optimization discussed before. It should be noted that, having the generally rapid OH bond oscillations on the one hand (promoting the direct PT path), and relatively small attractive O-N force on the other hand (promoting the indirect path), one should expect an interplay of the two relaxation mechanisms in the PPMet-4 system, which could be sensitive to the system environment.