A novel Transition-Based Constrained DFT (TCDFT) for the Robust and Reliable Treatment of Pure and Mixed Excitations in Molecular 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 typically computationally demanding and thus limited to moderate system sizes. We present a new variation of constrained DFT (CDFT), implemented in the BigDFT code, wherein the constraint corresponds to a particular transition (T) between occupied and virtual orbitals, rather than a region of the simulation space as in traditional CDFT. We perform benchmark T-CDFT calculations for a set of gas phase acene molecules and OLED emitters considering both pure and mixed excitation states. For both classes of molecules, we find that T-CDFT based on semi-local functionals is comparable to hybrid functional results from ΔSCF and TDDFT. Furthermore, T-CDFT proves to be more robust than ΔSCF and does not suffer from the well-known problems encountered when applying TDDFT to charge-transfer (CT) states, and is therefore applicable to both local excitations and CT states. Finally, T-CDFT is designed for large systems  and it is 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.