  Spontaneous emission and internal conversion rates are calculated within
harmonic approximations and compared to results obtained within the
semi-classical extended thawed Gaussian approximation. This is the first
application of the ETGA in the calculation of internal conversion and emission
rates for real molecular systems, namely formaldehyde, fluorobenzene, azulene
and a dicyano-squaraine dye. The viability of the models as black-box tools for
prediction of spontaneous emission and internal conversion rates is assessed.
All calculations were done using a consistent protocol in order to investigate
how different methods perform without previous experimental knowledge.
Contrasting the results with experimental data shows that there are further
improvements required before theoretical predictions of emission and internal
conversion rates can be used as reliable indicator for the photo-luminescence
properties of molecules. We find that the extended thawed Gaussian
approximation performs rather similar to the vertical harmonical model.
Including anharmonicities in the calculation of internal conversion rates has a
moderate effect on the quantitative results in the studied systems. The
electronic structure calculations were done using the B3LYP, PBE0,
$\omega$B97XD and CAM-B3LYP functionals. The choice of the functional does not
appear to be a major limiting factor for a black-box approach, when it comes to
the prediction of radiative and nonradiative rates for organic molecules. The
emission rates are fairly stable with respect to computational parameters, but
the internal conversion rate reveals itself to be highly dependent on the
choice of the spectral lineshape function, particularly the width of the
Lorentzian function, associated with homogeneous broadening.
