PAHs have been suggested as possible molecular carriers of the diffuse
interstellar bands (DIBs) that were first discovered in 1921.
Notwithstanding the significant efforts that have been devoted to this
problem in the past two decades, no individual PAHs have ever been
identified as DIB carriers due to the lack of a reliable method to
pre-select PAH candidates out of the infinite pool of PAH molecules. In
an attempt to solve this difficult PAH-DIB problem, I designed a novel
approach to generate a "complete" electronic database of PAHs. In the
first phase of this approach, a computer code is used to enumerate all
possible carbon skeletons of PAHs with a given number of fused benzene
rings. In the second phase, a ultrafast semi-empirical ab initio method
is employed to generate the vibronic energies and rotational constants
of all the PAHs generated in the phase one. An electronic database of
all PAHs with up to 10 fused benzene rings (~39,000 molecules) has been
generated this way. Possible PAH carriers are then pre-selected
according to their transition wavelengths, intensities, and the
simulated band profiles. High level quantum chemical calculations are
then performed to further narrow down the candidate list. A survey of
all PAHs in the current database shows that two closed-shell PAH cations
meet all constraints put on the strong lambda-^U6614 DIB (wavelength,
intensity, band profile, and ionization potential). This approach holds
great power in solving the PAH–DIB problem.