“…According to our survey of past research discussing spin-forbidden transitions in the context of NTOs, the latter were obtained either from 1TDMs between spin-free SR states belonging to different spin multiplicities, that is, without “integration” over spin degrees of freedom, or from SR spin-1TDMs. For recent publications, see, for example, refs , − , , , and . The NTOs thus obtained are clearly very useful for assigning a spin-forbidden transition.…”