The following isomers of
, depicted in
figure 8.3, were considered: a monocyclic ring, a flat
graphitic sheet, a bowl-shaped structure with one pentagon, and a
caged structure with a mixture of square, pentagonal and hexagonal
faces, and a fullerene. Other candidate structures, such as bicyclic
rings and a 3-pentagon bowl were excluded on the grounds that DFT
calculations using several different density functionals have shown
them to be significantly higher in
energy. [151,152]
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Three isomers of
were studied. A previous density
functional study [155] considered 10 isomers, but the
effects of basis set incompleteness are apparent and as only two
density functionals were used, the low energy ordering of the
structures is difficult to ascribe. Structures in the study included 3
graphitic sheet-like structures, a monocyclic
``cumulenic''8.1 ring, a cage
and a fullerene. The fullerene, ring and one of the sheets were
selected for DMC calculations due to their apparent low energy. The
fullerene is significantly distorted and has only
symmetry. The sheet resembles the purely graphitic
sheet but for the addition of a pentagon.
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Three structures of
were
investigated: a monocyclic ring, a graphitic sheet and a fullerene of
symmetry (figure 8.5). Other bowl and
sheet-like structures [156] were excluded on
energetic grounds.
Two
structures were examined: a monocyclic ring and
a fullerene (figure 8.6). HF calculations suggest that the
fullerene form of
is energetically
favourable, [146] demonstrating the reduced role
of electron correlation in determining energetic orderings with
increased cluster size. These structures were included to provide
additional information about trends in binding energies.