Evaluating the electronic structure of coexisting excitonic and multiexcitonic states in periodic systems: significance for singlet fission Xiaoyu Xie and Alessandro Troisi University of Liverpool, UK Singlet fission (SF) in organic solids is an example of processes that are challenging to describe with the most common electronic structure approaches. It involves optically bright singlet excited states delocalised over many molecules, which could be efficiently treated by density functional theory, and multi-excitonic localised states that have to be studied with wavefunction methods, usually with small clusters considering their expensive computational costs. We propose a methodology to combine multiconfigurational wavefunction calculations with reduced Hamiltonian to investigate the electronic structure of large clusters or fully periodic systems. The results allow one to study how states of different natures (excitonic, charge-transfer, and multiexcitonic) coexist and are contaminated by their couplings in large or periodic systems. Novel insights are therefore possible. For example, since the excitonic bands are relatively broad with respect to the multiexcitonic states, there are limited regions of the crystal momentum space where the transition between the two is more likely. References 1. Xiaoyu Xie, Alessandro Troisi, J. Chem. Theory Comput.2022, 18, 1, 394–405.
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