Thawed Gaussian Ehrenfest dynamics at conical intersections: When can a single mean-field trajectory capture internal conversion?

The thawed Gaussian Ehrenfest dynamics is a single-trajectory method that partially includes both nuclear quantum and electronically nonadiabatic effects by combining the thawed Gaussian wavepacket dynamics with Ehrenfest dynamics. First, we demonstrate the improvement over the parent methods in a multidimensional system consisting of vertically displaced harmonic potentials with constant diabatic couplings, for which the thawed Gaussian Ehrenfest dynamics is exact. Then, we show that single-trajectory mean-field methods completely fail to capture electronic population transfer in the vicinity of conical intersections between potential energy surfaces associated with electronic states of different symmetry (i.e., belonging to different irreducible representations of the molecular point group). The underlying cause of this limitation suggests that the thawed Gaussian Ehrenfest dynamics can be useful for studying nonadiabatic dynamics close to conical intersections of electronic states of the same symmetry, which have been understudied owing to the difficulty in locating them. Using a model of this type of intersection, we compare the thawed Gaussian Ehrenfest dynamics with exact quantum dynamics and find that the approximate mean-field approach yields a molecular wavefunction that remains qualitatively similar to the exact one even after crossing and recrossing the conical intersection.