Trivial Crossing Problem in FSSH: dissection of different methods on a weakly coupled spirobifluorene

Tully’s fewest switches surface hopping (FSSH) has been a popular method for simulating quantum-classical dynamics in a large variety of systems. Its popularity has also brought forward its limitations. Trivial unavoided crossings between non-interacting states that may occur in complex molecular systems present a huge numerical challenge in FSSH. At such crossings, the nonadiabatic (NA) coupling is infinite at the exact crossing point and vanishingly small elsewhere. Finite time-step numerical propagation is likely to miss such crossings and give erroneous results like unphysical long-range energy transfer. The correct treatment of trivial crossings is contingent on their proper identification. Here, we consider the performance of two different strategies developed to deal with the trivial crossing problem in FSSH. Specifically, (i) the Min-Cost adiabatic state identity tracking algorithm, and (ii) the self-consistent fewest switches surface hopping (SC-FSSH) approach have been implemented within the Non-adiabatic Excited State Molecular Dynamics (NA-ESMD) framework. The performance of the methods has been investigated for the internal conversion and energy transfer dynamics of a weakly coupled spiro-linked polyfluorene (spirobifluorene) system.