The spin contamination problem becomes severe for photochemical simulations and makes SF-TDDFT challenging to apply in a general way for NMD. In our experience, when starting from a triplet reference configuration, states that manifest as singlets upon vertical excitation ( Ŝ2 ≈ 0) quickly become highly mixed as the molecule moves away from the Frank-Condon region (typically Ŝ2 ≈ 1, in atomic units). As a result, it becomes difficult to distinguish singlets from triplets, and to proceed without human intervention requires some kind of a state-tracking procedure. Although some NMD simulations using SF-TDDFT have been reported using this approach, 21,39-46 a more reliable remedy is to remove the spin contamination directly. This can be done either exactly, by construction, or else approximately, but these procedures are not yet in widespread use.