Singlet-triplet transitions in three-atomic molecules studied by time-dependent MCSCF and density functional theory

Singlet-triplet transition moments and phosphorescence lifetimes have been calculated for the three-atomic molecules HCN, O₃, H₂O, H₂S, GeF₂, GeCl₂ and GeBr₂ by time-dependent density functional theory (DFT) utilizing quadratic response functions in order to qualify DFT which recently has become available for studies of this kind [TUNELL, I., RINKEVIVIUS, Z., VAHTRAS, O., SALEK, P., HELGAKER, T., and ÅOREN, H., 2003, J. chem. phys., 119, 11024]. Comparison with ab initio and experimental data indicates that DFT exhibit results of similar quality as explicitly correlated methods which indicates that it indeed is a viable approach for singlet-triplet transitions. O₃ provides an intriguing example in that a systematic investigation of the singlet-triplet transition moment of its Wulf band indicates a clear advantage of the DFT technique despite the multiconfigurational character of the electronic structure of this molecule. The electronic spin-spin coupling and the hyperfine nuclear coupling constants have also been calculated in order to further characterize the triplet state in the spectra of the investigated systems.