The color center singlet state of oxygen vacancies in TiO₂

Oxygen vacancies are ubiquitous in TiO₂ and play key roles in catalysis and magnetism applications. Despite being extensively investigated, the electronic structure of oxygen vacancies in TiO₂ remains controversial both experimentally and theoretically. Here, we report a study of a neutral oxygen vacancy in TiO₂ using state-of-the-art quantum chemical electronic structure methods. We find that the ground state is a color center singlet state in both the rutile and the anatase phases of TiO₂. Specifically, embedded coupled cluster with singles, doubles, and perturbative triples calculations find, for an oxygen vacancy in rutile, that the lowest triplet state energy is 0.6 eV above the singlet state, and in anatase, the triplet state energy is higher by 1.4 eV. Our study provides fresh insights into the electronic structure of the oxygen vacancy in TiO₂, clarifying earlier controversies and potentially inspiring future studies of defects with correlated wave function theories.