7.12.4 Excitonic Analysis of the RAS-CI Wave Function

The RAS-CI wave function of multi-chromophoric systems can be decomposed in terms of local excitations (LE), charge resonances (CR) and multi-exciton configurations (ME).

$$|\mathrm{\Psi }⟩=|{\mathrm{\Psi }}^{LE}⟩+|{\mathrm{\Psi }}^{CR}⟩+|{\mathrm{\Psi }}^{ME}⟩$$

(7.95)

The wave function decomposition scheme combines the use of fragment (localized) orbitals with the computation of charge and spin cumulant indexes.^{609, 130} Such analysis is extremely useful in order to characterize in great detail the nature of the ground and excited states in the presence of two or more molecules or chromophoric moieties. The ME contributions can be further decomposed into different spin coupled configurations localized on each. Typically, the ME part of a singlet state in a molecular dimer can be decomposed as singlet-singlet (SS) and triplet-tripled (TT) contributions.

$$|{\mathrm{\Psi }}^{ME}⟩\approx |{\mathrm{\Psi }}^{SS}⟩+|{\mathrm{\Psi }}^{TT}⟩$$

(7.96)

At the moment, this analysis is only available for the decomposition of spin singlet states.