Nucleation of small silicon carbide dust clusters in AGB stars
  Silicon carbide (SiC) grains are a major dust component in carbon-rich AGB
stars. The formation pathways of these grains are, however, not fully
understood.\ We calculate ground states and energetically low-lying structures
of (SiC)$_n$, $n=1,16$ clusters by means of simulated annealing (SA) and Monte
Carlo simulations of seed structures and subsequent quantum-mechanical
calculations on the density functional level of theory. We derive the infrared
(IR) spectra of these clusters and compare the IR signatures to observational
and laboratory data.\ According to energetic considerations, we evaluate the
viability of SiC cluster growth at several densities and temperatures,
characterising various locations and evolutionary states in circumstellar
envelopes.\ We discover new, energetically low-lying structures for
Si$_{4}$C$_{4}$, Si$_{5}$C$_{5}$, Si$_{15}$C$_{15}$ and Si$_{16}$C$_{16}$, and
new ground states for Si$_{10}$C$_{10}$ and Si$_{15}$C$_{15}$. The clusters
with carbon-segregated substructures tend to be more stable by 4-9 eV than
their bulk-like isomers with alternating Si-C bonds. However, we find ground
states with cage ("bucky"-like) geometries for Si$_{12}$C$_{12}$ and
Si$_{16}$C$_{16}$ and low-lying, stable cage structures for n $\ge$ 12. The
latter findings indicate thus a regime of clusters sizes that differs from
small clusters as well as from large-scale crystals. Thus, and owing to their
stability and geometry, the latter clusters may mark a transition from a
quantum-confined cluster regime to crystalline, solid bulk-material.
  The calculated vibrational IR spectra of the ground-state SiC clusters shows
significant emission. They include the 10-13 $\mu$m wavelength range and the
11.3 $\mu$m feature inferred from laboratory measurements and observations,
respectively, though the overall intensities are rather low.
