The concept of an "exciton" or bound particle/hole pair is ubiquitous in solid-state physics yet it can be difficult to connect that language to the MO-based concepts that are used in quantum chemistry, since any excited state consists of an excited electron and a hole in the occupied space. A connection can be made by identifying virtual-occupied function pairs ψ a (r elec ) ψ * i (r hole ) as a quasiparticle basis. The transition density T (r, r ′ ) in Eq. (2.5), written in the form T (r elec , r hole ), is then identified as an electron/ hole "wave function" for the exciton. That said, the true excited-state wave function in a many-body formalism is Ψ exc in Eq. (2.8), which is be used to construct T (r, r ′ ). Nevertheless, the transition density T (r elec , r hole ) is often called a "wave function" in quasiparticle theories based on the two-particle Green's function and the Bethe-Salpeter equation. Semantics aside, this quantity facilitates examination of the spatial correlation and separation between particle and hole. In quantum chemistry, this form of analysis was pioneered by Mukamel, Tretiak, and Chernyak, and later used by others, mostly in the context of organic photovoltaic materials and using semi-empirical CIS-type wave functions. These ideas were subsequently formalized, and generalized to wave functions of arbitrary complexity, and to the case of nonorthogonal basis functions, by Plasser and co-workers. Those authors also studied organic photovoltaics, albeit using TD-DFT and correlated wave functions rather than semi-empirical methods.