Vibronic dynamics from real-time time-dependent density-functional theory coupled to the Ehrenfest scheme: the example of p-coumaric acid

Abstract We investigate the vibronic dynamics of a modified version of the p-coumaric acid using real-time time-dependent density-functional theory coupled with the Ehrenfest scheme in the adiabatic local density approximation. Due to the issues of this functional to yield a reliable starting point for the evolution of the electron-nuclear system triggered by a pulse, we start off the simulations constraining the electronic occupation of the molecule in two excited states corresponding to a bright, delocalized transition, and a dark, charge-transfer-like excitation. By monitoring the kinetic energy spectral density, we analyze the nature of the nuclear motion over a time window of 300 fs. Anharmonic effects appear at low frequencies, below 500 cm $$^{-1}$$ - 1 , and are particularly pronounced in the charge-transfer excitation. In this case, after about 200 fs, the molecular backbone becomes largely distorted and the initially constrained occupations evolve toward a different electronic configuration. On the other hand, the dynamics initialized from the delocalized bright excitation are electronically and structurally stable, and the resulting nuclear motion is markedly harmonic. Our results provide indications to decipher the vibronic dynamics of this chromophore and related systems in view of more elaborated simulations embedding the molecules in a realistic environment.