Density functional theory (DFT) and time-dependent DFT calculations have been performed on a set of 34 titanium dioxide clusters ((TiO2) n with n 125) to investigate structural and electronic properties of nanostructured TiO 2 (nano-TiO 2 ) materials. The investigated clusters include models of the three low-energy polymorphic forms of TiO 2 anatase, rutile, and brookite. A systematic comparison of clusters of increasing size show clear trends for emerging bulk properties in the investigated systems as the surface-to-bulk ratio changes from small clusters dominated by undercoordinated surface atoms to more realistic model nano-crystals with significant bulk components. Differences and similarities in terms of atomic coordination, structural stability, and electronic properties for the three different polymorphic forms of nano-TiO 2 are discussed. The calculations provide evidence for emerging polymorphism with increasing cluster sizes so that the different TiO 2 forms can be clearly distinguished based on structural characteristics associated with the local bonding environment of the constituent atoms.
The electron-detachment energy is measured by its ionization potential (IP). As a result, it is a fundamental observable and important molecular electronic signature in photoelectron spectroscopy. A precise theoretical prediction...
Recent Rydberg spectroscopy
measurements of a diamine molecule,
N
,
N
′-dimethylpiperazine (DMP),
indicate the existence of a localized electronic state as well as
a delocalized electronic state. This implies that the cation, DMP
+
, can similarly have its positive charge either localized
on one of the N atoms or delocalized over both. This interpretation
of the experiments has, however, been questioned based on coupled
cluster calculations. In this article, results of high-level multireference
configuration interaction calculations are presented where a localized
state of DMP
+
is indeed found to be present with an energy
barrier separating it from the delocalized state. The energy difference
between the two states is in excellent agreement with the experimental
estimate. The results presented here, therefore, support the original
interpretation of the experiments and illustrate a rare shortcoming
of CCSD(T), the “gold standard” of quantum chemistry.
These results have implications for the development of density functionals,
as most functionals fail to produce the localized state.
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