  In this work we characterize an initial van der Waals adduct in the potential
energy surface of reaction between cyanoacetylene HC3N and the cyano radical.
The geometry of the CN-HC3N adduct has been optimized through calculations
employing ab initio methods. Results show that the energy of the adduct lays
below the reactants. Additionally, a saddle point that connects the adduct to
an important intermediate of the PES has been localized, with energy below the
reactants. Calculations of the intermolecular potential have been performed and
results show that the energy of the van der Waals adduct is higher than
estimated with the ab initio methods.
