ABSTRACT
When a thermally equilibrated chromophore is promoted to an excited electronic state by light absorption,nuclearmotionistriggered.§eevolutionoftheoriginalmolecularandelectronicstructures determinesthefateoftheabsorbedlightenergythatcaneitherbereleased(viainternalconversion/ luminescence)orexploitedtodriveachangeintheoriginalchromophorestructure(i.e.,aphotochemicalreaction).§roughouttheseprocesses,theinitiallypopulatedFranck-Condon(FC)activemodes coupletootheravailablenormalmodes,thiscouplingrenderedevenmoree¨ectivebytheanharmonicityofmolecularvibrations.1,2 Computational chemists view such events in terms of the evolution of the chromophore’snuclearcoordinatesondi¨erent3N-6dimensionalpotentialenergysurfaces,whereNis thenumberofatoms.§esearerigorouslyde©nedonthebasisoftheBorn-Oppenheimer(BO)approximation,3,4 which states that the relatively large mass of a nucleus, as compared to that of an electron, permitstheseparationofelectronicandnuclearmotions.AconsequenceoftheBOapproximationis thatforanymolecularsystem,thereexistsadi¨erentpotentialenergysurfaceforeachelectronicstate. Sinceintheseprocesses,theinitialexcitedstateelectronicstructureultimatelyevolvesintoaground stateelectronicstructure,thecomputationaldescriptionoflight-energywastageandexploitationatthe molecularlevelrequiresthemappingofdi¨erentpotentialenergysurfaces.Mostimportantly,onehas to attain an understanding of how and where potential energy surfaces connect.
