  Computational modeling of titanium dioxide nanoparticles of realistic size is
extremely relevant for the direct comparison with experiments but it is also a
rather demanding task. We have recently worked on a multistep/scale procedure
to obtain global optimized minimum structures for chemically stable spherical
titania nanoparticles of increasing size, with diameter from 1.5 nm (~300
atoms) to 4.4 nm (~4000 atoms). We use first self-consistent-charge density
functional tight-binding (SCC-DFTB) methodology to perform thermal annealing
simulations to obtain globally optimized structures and then hybrid density
functional theory (DFT) to refine them and to achieve high accuracy in the
description of structural and electronic properties. This allows also to assess
SCC-DFTB performance in comparison with DFT(B3LYP) results. As a further step,
we investigate photoexcitation and photoemission processes involving
electron/hole pair formation, separation, trapping and recombination in the
nanosphere of medium size by hybrid DFT. Finally, we show how a recently
defined new set of parameters for SCC-DFTB allows for a proper description of
titania/water multilayers interface, which paves the way for modeling large
realistic nanoparticles in aqueous environment.
