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Non-Orthogonal Exciton Theory Including Charge Transfer Effects for Strongly Coupled Chromophores

Year: 2026
Type: preprint
Abstract: Frenkel exciton models are popular ways to describing non-covalently bound multichromophore systems in large molecules and molecular aggregates, where system size poses a challenge for mainstream quantum-chemical methods. While fragments can conveniently be described with embedding methods at the desired level of sophistication, the proper description of the interaction of the chromophores requires accurate coupling between them. At large distances it is dominated by the Coulomb-type term, but at short separations, the exchange contribution, the overlap of the wave functions, and the charge transfer (CT) effects also play an important role. In this work a strategy for evaluating the latter terms, by exploiting second-quantized formalism with non-orthogonal orbitals, is presented. The derived formulae use quantities from the monomer calculations, such as transition densities for local excitations and Dyson orbitals of ionized species for CT states. This ensures that the framework can be used with a wide spectrum of electronic structure methods, including high-level EOM-CC variants. The importance of the different contributions as functions of the intermolecular separation is tested on the formaldehyde dimer. While all terms become important with decreasing distance, their actual magnitude cannot be predicted without calculations. The robust accuracy of the proposed exciton model down to the 2 Å intermonomer separation is shown for the eight lowest lying excited states of the formaldehyde dimer, by comparing the potential curves obtained from dimer calculations and those from the presented model involving ten electronic states.
Source: ChemRxiv
DOI: 10.26434/chemrxiv.15008074/v1
Language: English
Open Access status: green
OpenAlex ID: W7204838278
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