A Nonadiabatic Excited State Molecular Mechanics/Extended Hückel Ehrenfest Method

In this work we study intramolecular vibronic relaxation effects and photoisomerization dynamics in atomistic molecular systems by means of a mixed self-consistent quantum–classical Ehrenfest formalism for nonadiabatic molecular dynamics (NA-MD) in the excited state. The quantum mechanical part of the method is based on the extended Hückel formalism, whereas the nuclei are treated with the molecular mechanics method. The self-consistent coupling between quantum and classical degrees of freedom is achieved by nonadiabatic Hellmann–Feynman–Pulay forces that conserve the total (quantum–classical) energy. Moreover, this work demonstrates the capabilities of a method that combines two simple though efficient computational schemes to describe complex excited state effects such as photoisomerization dynamics of stilbene and azobenzene molecules. In particular, for stilbene we point out the key importance of hydrogen out-of-plane (HOOP) modes to trigger the isomerization of the phenyl rings. For azobenzene, we identify the inversion-assisted rotation and the associated pedal-cycle motion of the CNNC dihedral as the driving isomerization mechanism. The method can also be applied for NA-MD studies of molecules adsorbed on extended solid surfaces and charge transfer processes