  A four-dimensional potential energy surface for the interaction between He
and HCN with vibrational bending is presented. Ab initio calculations were
carried out at the coupled-cluster level with single and double excitations and
a perturbative treatment of triple excitations, using a quadruple-zeta basis
set and mid-bond functions. An accurate fit to the Ab-initio data has been
obtained with a parametrized functional form. The global minimum is found in
the linear He-HCN configuration with the H atom pointing towards Helium at the
intermolecular separation of 7.94 a0. The corresponding well depth is 30.35
cm^-1 . The quality of the new potential has been tested by performing two
comparisons with previous theoretical and experimental works. First, the
rovibrational energy levels of the He-HCN complex for a linear configuration of
the HCN molecule have been calculated. The dissociation energy is 8.99 cm^-1,
which is slightly smaller than the semi-empirical value of 9.42 cm^-1 . The
transitions frequencies are found to be in good agreement with the experimental
data. Second, we performed close coupling calculations of the rotational
de-excitation of HCN in collisions with He and observed a close similarity with
the theoretical data published in a recent study.
