Excitation energies of some d1systems calculated using time-dependent density functional theory: an implementation of open-shell TDDFT theory for doublet–doublet excitations

Time-dependent density functional theory has been used extensively to study the excitation energies of closed-shell systems with great success, while its applications to open-shell systems are still limited, especially open-shell transition metal compounds. In the present work, the excitation energies of ligand to metal charge transfer excitations and d→d excitations of some open-shell d1 MLX4 transition metal compounds with spin-unrestricted TDDFT were studied. The results demonstrate that TDDFT can be used to study the excitation energies of the title systems accurately. With BP86/ALDA, the errors are mostly around 0.3–0.5 eV, which is similar to the error for TDDFT calculations on closed-shell systems.