Transition-Based Constrained DFT for the Robust and Reliable Treatment of Excitations in Supramolecular Systems

Despite the variety of available computational approaches, state-of-the-art methods for calculating excitationenergies, such as time-dependent density functional theory(TDDFT), are computationally demanding and thus limited tomoderate system sizes. Here, we introduce a new variation ofconstrained DFT (CDFT), wherein the constraint corresponds to aparticular transition (T), or a combination of transitions, betweenoccupied and virtual orbitals, rather than a region of the simulationspace as in traditional CDFT. We compare T-CDFT with TDDFTand Delta SCF results for the low-lying excited states (S1and T1)ofaset of gas-phase acene molecules and OLED emitters and withreference results from the literature. At the PBE level of theory, T-CDFT outperforms Delta SCF for both classes of molecules, while alsoproving to be more robust. For the local excitations seen in the acenes, T-CDFT and TDDFT perform equally well. For the chargetransfer (CT)-like excitations seen in the OLED molecules, T-CDFT also performs well, in contrast to the severe energyunderestimation seen with TDDFT. In other words, T-CDFT is equally applicable to both local excitations and CT states, providingmore reliable excitation energies at a much lower computational cost than TDDFT cost. T-CDFT is designed for large systems andhas been implemented in the linear-scaling BigDFT code. It is therefore ideally suited for exploring the effects of explicitenvironments on excitation energies, paving the way for future simulations of excited states in complex realistic morphologies, such as those which occur in OLED materials