Much of the work on CT metrics for TD-DFT has been carried out by Ciofini and coworkers. Without referring to them as such, these authors introduce particle and hole densities (or attachment and detachment densities, which are equivalent within TD-DFT as discussed in Section 2.2). Because these quantities describe the parts of space that are characterized by positive (∆ρ elec ) and negative (∆ρ hole ) changes in the density, these density changes were called ρ + (r) and ρ -(r), (