Describing
Excited State Relaxation and Localization
in TiO<sub>2</sub> Nanoparticles Using TD-DFT
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Abstract
We have investigated the description
of excited state relaxation
in naked and hydrated TiO<sub>2</sub> nanoparticles using Time-Dependent
Density Functional Theory (TD-DFT) with three common hybrid exchange-correlation
(XC) potentials: B3LYP, CAM-B3LYP and BHLYP. Use of TD-CAM-B3LYP and
TD-BHLYP yields qualitatively similar results for all structures,
which are also consistent with predictions of coupled-cluster theory
for small particles. TD-B3LYP, in contrast, is found to make rather
different predictions; including apparent conical intersections for
certain particles that are not observed with TD-CAM-B3LYP nor with
TD-BHLYP. In line with our previous observations for vertical excitations,
the issue with TD-B3LYP appears to be the inherent tendency of TD-B3LYP,
and other XC potentials with no or a low percentage of Hartree–Fock
like exchange, to spuriously stabilize the energy of charge-transfer
(CT) states. Even in the case of hydrated particles, for which vertical
excitations are generally well described with all XC potentials, the
use of TD-B3LYP appears to result in CT problems during excited state
relaxation for certain particles. We hypothesize that the spurious
stabilization of CT states by TD-B3LYP even may drive the excited
state optimizations to different excited state geometries from those
obtained using TD-CAM-B3LYP or TD-BHLYP. Finally, focusing on the
TD-CAM-B3LYP and TD-BHLYP results, excited state relaxation in small
naked and hydrated TiO<sub>2</sub> nanoparticles is predicted to be
associated with a large Stokes’ shift