Choice of the electronic basis for field-induced surface hopping

A method combining an Ehrenfest-like approach, which minimizes the number of surface hops, with Tully's fewest switches surface hopping is proposed for the description of nonadiabatic molecular dynamics in the presence of an external electromagnetic field. When two states are strongly coupled by the applied light field, an Ehrenfest-like approach is employed, which allows the system to be in a coherent superposition. Tully's fewest switches surface hopping (FSSH) is used for the weak- and no-coupling regimes to improve the asymptotic behavior of the method. The decision of which approach to employ at a given time is made based on a simple analysis of Rabi oscillations in a two-state model. The method is tested for two exactly solvable model systems, i.e., a stimulated emission scenario in a pulsed laser field and a photoexcitation scenario in a cw laser field. Position and momentum densities of the nuclei compare well with exact quantum dynamics simulations and improve on both a pure Ehrenfest and a pure FSSH approach. The method is efficient and easily implemented.