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 excitation energies such as time-dependent density functional theory (TDDFT), are computationally demanding and thus limited to moderate system sizes. Here, we introduce a new variation of constrained DFT (CDFT), wherein the constraint corresponds to a particular transition (T), or combination of transitions, between occupied and virtual orbitals, rather than a region of the simulation space as in traditional CDFT. We compare T-<PRE_TAG>CDFT</POST_TAG> with TDDFT and DeltaSCF results for the low lying excited states (S_{1} and T_{1}) of a set of gas phase acene molecules and OLED emitters, as well as with reference results from the literature. At the PBE level of theory, T-<PRE_TAG>CDFT</POST_TAG> outperforms DeltaSCF for both classes of molecules, while also proving to be more robust. For the local excitations seen in the acenes, T-<PRE_TAG>CDFT</POST_TAG> and TDDFT perform equally well. For the charge-transfer (CT)-like excitations seen in the OLED molecules, T-<PRE_TAG>CDFT</POST_TAG> also performs well, in contrast to the severe energy underestimation seen with TDDFT. In other words, T-<PRE_TAG>CDFT</POST_TAG> is equally applicable to both local excitations and CT states, providing more reliable excitation energies at a much lower computational cost than TDDFT. T-<PRE_TAG>CDFT</POST_TAG> is designed for large systems and has been implemented in the linear scaling BigDFT code. It is therefore ideally suited for exploring the effects of explicit environments on excitation energies, paving the way for future simulations of excited states in complex realistic morphologies, such as those which occur in OLED materials.