Validating Fewest-Switches Surface Hopping in the Presence of Laser Fields

Abstract

The capability of fewest-switches surface hopping (FSSH) to describe non-adiabatic dynamics of small and medium sized molecules under explicit excitation with external fields is evaluated. Different parameters in FSSH and combinations thereof are benchmarked against multi-configurational time dependent Hartree (MCTDH) reference calculations using SO$_2$ and 2-thiocytosine as model, yet realistic, molecular systems. Qualitatively, we find that FSSH is able to reproduce the trends in the MCTDH dynamics with (and without) an explicit external field; however, no set of FSSH parameters is ideal. An adequate treatment of the overcoherence in FSSH is identified as the driving factor to improve the description of the excitation process with respect to the MCTDH reference. Here two corrections were tested, the augmented-FSSH (AFSSH) and the energy-based decoherence correction. A dependence on the employed basis is detected for the AFSSH algorithm, performing better when spin-orbit and external laser field couplings are treated as off-diagonal elements instead of projecting them onto the diagonal of the Hamilton operator. In the presence of an electric field, the excited state dynamics was found to depend strongly on the vector used to rescale the kinetic energy along after a transition between surfaces. For SO$_2$, recurrence of the excited wave packet throughout the duration of the applied laser pulse is observed for long laser pulses (>100~fs), resulting in additional interferences not captured by FSSH and only visible in variational multi-configurational Gaussian when utilizing a large amount of gaussian basis functions. This feature essentially vanishes when going towards larger molecules, such as 2-thiocytosine, where this effect is barely visible in a laser pulse with a full width at half maximum of 200~fs.