Five two-dimensional potential energy surfaces for the interaction of He with cyanoacetylene (HCCCN) are presented, obtained from ab initio calculations using symmetry-adapted perturbation theory and the supermolecular method at different levels of electron correlation. HCCCN is taken to be a rigid linear molecule with the interatomic distances fixed at the experimental "r(0)" geometry extracted from ground-state rotational constants. The complex was found to have a global minimum at a T-shaped configuration and a secondary minimum at the linear configuration with the He atom facing the H atom. Two saddle points were also located. There is good agreement between the positions of the stationary points on each of the five surfaces though their energies differ by up to 19%. Rovibrational bound state calculations were performed for the He-4-HCCCN and He-3-HCCCN complexes. Spectra (including intensities) and wave functions of He-4-HCCCN obtained from these calculations are presented. The effective rotational constant of HCCCN solvated in a helium droplet was estimated by minimizing the energy of He-n-HCCCN for n=2-12, selecting the n=7 complex as giving the largest magnitude of interaction energy per He, and shifting the resulting ring of He atoms to the position corresponding to the average geometry of the ground state of the He-HCCCN dimer. This estimate is within 4.8% of the measured value. (C) 2003 American Institute of Physics.