  The present work purports to identify candidate carriers of the UIBs. This
requires a procedure for the computation of the emission spectrum of any given
candidate. The procedure used here consists in exciting the carrier into a
state of internal vibration, waiting until the system has reached dynamic
equilibrium and, then, monitoring the time variations of the overall electric
dipole moment associated with this vibration. The emission spectrum is shown to
be simply related to the FT of these variations. This procedure was applied to
more than 100 different chemical structures, inspired by the exhaustive
experimental and theoretical analyses of Kerogens, the terrestrial sedimentary
matter, which is known to be mainly composed of C, H, O, N and S atoms. From
this data base, 21 structures were extracted, which fall in 4 classes, each of
which contributes preferentially to one of the main UIBs. Summing their
adequately weighted spectra delivers an emission spectrum which indeed exhibits
the main UIB features (allowing for computational errors inherent in the
chemical simulation code). By changing the weights, it is possible to change
the relative band intensities so as to mimic the corresponding (moderate)
changes observed in the sky. The defects of the present simulation are
discussed, and directions for improvement are explored.
