PN is a scalar ranging from 1 (complete exciton localization) to 4 (complete exciton delocalization). We note that the quantity given by eq. ( ) (or equations of a similar form) is sometimes called "inverse participation ratio (IPR)", as was also done in our previous works. However, as recently highlighted by Herbert, the term IPR may lead to a confusion. Indeed, following ref. Tretiak and co-workers, we will call the quantity defined by eq. ( ) "participation number" (or "PN") to avoid possible confusions associated with the "IPR".