Tuning range-separated DFT functionals for accurate orbital energy modeling of conjugated molecules

Density functional theory (DFT) calculations with range-separated (RS) functionals are important for orbital energy modeling of conjugated molecules that involve charge transfer excitation. However, the accuracy of the computed results depends on the range-separation parameter (w), and the optimal values of w for a wide range of conjugated systems has hitherto been missing. Herein, orbital energy modeling of twelve representative conjugated molecules, that are promising electron-donors in bulk heterojunction-based organic photovoltaic devices, are benchmarked using DFT and time-dependent DFT (TD-DFT) with three RS functionals (LC-BLYP, wB97XD and CAM-B3LYP). The results using the RS functionals under consideration with default values of w deviate largely from the experimental values (mean signed error (MSE) on HOMO are 2.1eV, 0.93eV and 1.5eV, and MSE on vertical excitation energies are 0.47eV, 0.55eV and 0.82eV, respectively). Computation of orbital energies using tuned range-separation parameter for these RS functionals in the range of 0.05≤w≥0.5Bohr-1 indicates that w plays a significant role on the accuracy of the ground and the excited state energies. We found that the accurate orbital energies of conjugated systems can be predicted using values of w between 0.1 and 0.15Bohr-1, much smaller than the default values of 0.47, 0.33 and 0.20Bohr-1 used in LC-BLYP, CAM-B3LYP and wB97XD, respectively.