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Nonorthogonal configuration interaction to study electron and excitation energy transfer

PhD ceremony:Mr R.K. (R K) Kathir
When:January 18, 2022
Supervisors:C. (Coen) de Graaf, prof. dr. H.B. (Ria) Braam
Co-supervisor:dr. R.W.A. (Remco) Havenith
Where:Academy building RUG
Faculty:Science and Engineering

A theoretical study of electron transfer (ET) and excitation energy transfer (EET) processes can provide a more nuanced understanding of the underlying physics that may not always be accessible through experiments. The focus of this thesis is the development of nonorthogonal configuration interaction (NOCI) - Fragments, an electronic structure method that has shown promise for the study of ET and EET phenomena. The main aspects of NOCI-Fragments are: the possibility to retain the diabatic nature of the involved molecular states, a compact expansion of the NOCI wavefunction in terms of many-electron basis functions (MEBFs) which are spin-adapted antisymmetrized products of molecular (multiconfigurational) wavefunctions, direct accessibility of electronic coupling between diabatic states, and the inclusion of static correlation effects. NOCI is therefore a viable option to give an unbiased description of the ground and excited-state wavefunctions in a molecular cluster. Singlet fission (SF) is a widely studied EET process due to the promise it holds to improve organic photovoltaic efficiency and was chosen as a suitable application to be investigated using NOCI-Fragments. The work done in the thesis makes it possible for NOCI-Fragments to: study large molecular systems that are relevant for ET and EET processes, using multiconfigurational wavefunctions with reasonably large active spaces as the initial molecular wavefunctions, include dynamical correlation effects within each molecule, and include effects of the environment. The advancements from this thesis will prove useful in providing a clearer and realistic view of some photophysical phenomena.