Excited-State Molecular Dynamics Triggered by Light Pulses- Ab Initio Multiple Spawning vs Trajectory Surface Hopping.

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Trajectory surface hopping (TSH) and ab initio multiple spawning are two commonly employed methods for simulating the excited-state dynamics of molecules. TSH portrays the dynamics of nuclear wavepackets by a swarm of independent classical trajectories, which can hop between electronic states. Ab initio multiple spawning, however, expresses nuclear wavepackets on the basis of traveling, coupled basis functions, whose number can be extended in the case of coupling between electronic states. In the following, we propose to compare the performance of these two methods to describe processes involving the explicit interaction of a molecule with laser pulses. We base this comparison on the LiH molecule, as it is compatible with numerically exact simulations using quantum dynamics. As recognized in earlier works, the limitations of TSH due to its inherent independent trajectory approximation are further enhanced when studying an explicit photoexcitation. While ab initio multiple spawning is also based on a series of approximations, the couplings between its traveling basis functions allow for a proper description of phenomena that TSH cannot describe with its inherent independent trajectory approximation, even when applying decoherence corrections. We show here for different in silico experiments involving laser pulses that ab initio multiple spawning overcomes the limitations experienced by TSH and offers an at least qualitative description of population transfer between electronic states.