Do Anionic Titanium Dioxide Nano-Clusters Reach Bulk Band Gap? A Density Functional Theory Study

The electronic properties of both neutral and anionic (TiO2)n (n = 1-10) clusters are investigated by extensive density functional theory calculations. The predicted electron detachment energies and excitation gaps of anionic clusters agree well with the original experimental anion photoelectron spectra (APES). It is shown that the old way to analyze APES tends to overestimate vertical excitation gaps (VGA) of large anionic clusters, due to the nature of multiple electronic origins for the higher APES bands. Moreover, the VGA of anionic TiO2 clusters are evidently smaller than those of neutral clusters, which may also be the case for other metal oxide clusters with high electron affinity

Correlated electronic structure theory for challenging systems

The photochemistry of molecules can be investigated computationally, and this provides great insight into the underlying chemistry and physics. Such computational approaches are challenging and can pose many difficulties compared to ground state methodologies. Care must be taken to accurately describe these systems, as some lowlevel approximate methods can fail. The geometrical and electronic structures (TiO2)n clusters (n=1-4) have been investigated. These are of enormous technological interest as wide band-gap semiconductors yet the nature of electronic transitions in nano-sized clusters has yet to be fully elucidated. Structures of the neutral closed-shell, radical cationic and radical anionic clusters at each size are described and rationalised in terms of the pseudo-Jahn- Teller effect. We have used high-level response theory to set benchmarks for such systems. The TiO2 monomer is the simplest of the clusters studied yet proves a stern test for many lower order ab-initio methods. It is shown that high-level methods are required to properly describe this simple molecule. The Monte Carlo Configuration Interaction method attempts to combine the power of Full CI with a scalability that allows it to be used to study much larger systems. It can be systematically improved and can approach the accuracy of the Full CI method. This method is applied here to investigate potential energy surfaces and multipole moments of a range of small but challenging systems