Chart II: The electronic Hamiltonian of PDI-F tetramer in the diabatic representation. To reproduce the calculated low-lying electronic absorption spectra by TD-DFT method, we select 14 lowest-energy diabatic states across two categories of excitons (LEs, and CTEs between nearest neighbors and next nearest neighbors) to construct the electronic Hamiltonian of tetrameters as shown in Charts I and II. These electronic Hamiltonians are obtained by the adiabatic-to-diabatic (ATD) transformation procedure of FPHD approach. More computational procedures can be checked in SI. As Figure shows, the spectra yielded by the mixed LE-CTE exciton model with LE-CTE coupling agree well with these from the TD-DFT calculation, indicating that the chosen number of quasi-diabatic states is sufficient to accurately describe the lowest-energy adiabatic states. Three low-energy absorption peaks are observed.