  We present a rare event sampling scheme applicable to coupled electronic
excited states. In particular, we extend the forward flux sampling (FFS) method
for rare event sampling to a nonadiabatic version (NAFFS) that uses the
trajectory surface hopping (TSH) method for nonadiabatic dynamics. NAFFS is
applied to two dynamically relevant excited-state models that feature an
avoided crossing and a conical intersection with tunable parameters. We
investigate how nonadiabatic couplings, temperature, and reaction barriers
aspect transition rate constants in regimes that cannot be otherwise obtained
with plain, traditional TSH. The comparison with reference brute-force TSH
simulations for limiting cases of rareness shows that NAFFS can be several
orders of magnitude cheaper than conventional TSH, and thus represents a
conceptually novel tool to extend excited-state dynamics to time scales that
are able to capture rare nonadiabatic events.
