  Recent astronomical observations revealed an increasing molecular complexity
in the interstellar medium through the detection of a series of large cyclic
carbon species. To correctly interpret these detections, a complex analysis is
necessary that takes into account the non-local thermodynamic equilibrium
(non-LTE) conditions of the emitting media (e.g. when energy level populations
deviate from a Boltzman distribution). This requires proper state-to-state
collisional data for the excitation and de-excitation processes of the
molecular levels. Cyclopentadiene (c-C5H6), which was recently detected in
multiple cold interstellar clouds, is extensively studied in many aspects due
to its large importance for chemistry in general. At the same time, there are
no collisional data available for this species, which are necessary for a more
precise interpretation of the corresponding detections. In this work, we first
provide an accurate 3D rigid-rotor interaction potential for the [c-C5H6 + He]
complex from high-level of ab initio theories, which has been used to study
their inelastic collision by the exact close coupling quantum scattering
method. To the best of our knowledge, this is the first study where this method
is systematically applied to treat the dynamics of molecular collisions
involving more than ten atoms. We also analyse the collisional propensity rules
and the differences in contrast to calculations, where the approximate coupled
states scattering methods is used.
