These are the root-mean-square (RMS) sizes of the excited electron and the hole, respectively. Two other quantities worth mentioning are the "Hindex" and the "t-index" introduced by Ciofini and coworkers. The former is essentially (σ elec + σ hole )/2 but restricted to a one-dimensional donor-acceptor coordinate, while t = D CT -H. These quantities will not be addressed directly in the present work, as they do not appear to provide new information beyond what is obtainable using the readily interpretable expectation values defined above, combined in various ways.