  TiO2 nanoparticles (NPs) are nowadays considered fundamental building blocks
for many technological applications. Morphology is found to play a key role
with spherical NPs presenting higher binding properties and chemical activity.
From the experimental point of view, the characterization of these nano-objects
is extremely complex, opening a large room for computational investigations. In
this work, TiO2 spherical NPs of different size (from 300 to 4000 atoms) have
been studied with a two-scale computational approach. Global optimization to
obtain stable and equilibrated NSs was performed with a self-consistent charge
density functional tight-binding (SCC-DFTB) simulated annealing process,
causing a considerable atomic rearrangement within the nanospheres. Those
SCC-DFTB relaxed structures have been then optimized at DFT(B3LYP) level of
theory. We present a systematic and comparative SCC-DFTB vs DFT(B3LYP) study of
the structural properties, with particular emphasis on the surface-to-bulk
sites ratio, coordination distribution of surface sites and surface energy.
From the electronic point of view, we compare HOMO-LUMO and Kohn-Sham gaps,
total and projected density of states. Overall, the comparisons between DFTB
and hybrid DFT show that DFTB provides a rather accurate geometrical and
electronic description of these nanospheres of realistic size (up to a diameter
of 4.4 nm) at an extremely reduced computational cost. This opens for new
challenges in simulations of very large systems and more extended molecular
dynamics.
