The Structure of the Electronic Excitation of Molecules in Quantum-chemical Models

Methods for the analysis of the excited states of molecules and their electronic distribution are examined. The advantages of using procedures based on transition density matrices are noted. The application of the delocalisation numbers of the transition, reflecting the multiconfiguration nature of the electronically excited state, the excitation localisation numbers, providing a quantitative measure of the localisation of the transition in a particular molecular fragment, the interfragment charge transfer numbers, the atomic excitation energy indices, indicating the fraction of the molecular excitation energy which can be assigned to a given atom, etc., all of which have been introduced recently, are discussed. The structural-chemical factors favouring the isolation of the excitation of a single fragment are considered.
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