Vibronic coupling effects on the structure and spectroscopy of neutral and charged TiO2 clusters

Highlights: ► Vibronic coupling between non-degenerate states responsible for lower symmetry geometries in neutral and charged TiO2 clusters. ► CASSCF used to determine pseudo-Jahn–Teller couplings. ► Spurious symmetry breaking with different DFT functionals. ► B3LYP performs reasonably well, but care must be taken when studying charged TiO2 radicals with DFT. -- Abstract: We present a detailed computational study of the structure of small (TiO2)n nanoclusters including neutral closed-shell, radical cation, and radical anion species. We show that the geometries can be rationalised in terms of the pseudo-Jahn–Teller effect. A variety of methods are employed including CASSCF, DFT (and TD-DFT), EOM-CCSD, and Bruekner doubles (BD). The CASSCF method uses symmetry restrictions on analytical Hessians and can be seen as benchmark results for the smallest systems. Using these for calibration we then show that in general the B3LYP functional in conjunction with a reasonably large all electron basis is reasonably accurate, and in conjunction with TD-B3LYP can give a qualitative picture of the underlying pseudo-Jahn–Teller vibronic coupling effects. We do however show that artificial symmetry breaking does occur for some radical clusters, and that different DFT functionals unfortunately display this in a non-systematic manner.