  We have computed the linear optical absorption spectra of three graphene
quantum dots (GQDs), saturated by hydrogens on the edges, using both
first-principles time-dependent density-functional theory (TDDFT) and the
Pariser-Parr-Pople (PPP) model coupled with the configuration-interaction (CI)
approach. To understand the influence of electron-correlation effects, we have
also calculated the singlet-triplet energy gap (spin gap) of the three GQDs.
Because of the presence of edge hydrogens, these GQDs are effectively
polycyclic aromatic hydrocarbons (PAHs) dibenzo[bc,ef]coronene (also known as
benzo(1,14)bisanthene, C$_{30}$H$_{14}$), and two isomeric compounds,
dinaphtho[8,1,2abc;2,1,8klm]coronene and dinaphtho[8,1,2abc;2,1,8jkl]coronene
with the chemical formula C$_{36}$H$_{16}$. The two isomers have different
point group symmetries, $C_{2v}$, and $C_{2h}$, therefore, this study will also
help us understand the influence of symmetry on optical properties. A common
feature of the absorption spectra of the three GQDs is that the first peak
representing the optical gap is of low to moderate intensity, while the intense
peaks appear at higher energies. For each GQD, PPP model calculations performed
with the screened parameters agree well with the experimental results of the
corresponding PAH, and also with the TDDFT calculations.
