A single-valued double many-body expansion potential energy surface is reported for ground-state HCN based on a realistic representation of the long-range forces, and a fit to accurate ab initio calculations for the $\mathrm{HCN\leftrightarrow CNH}$ regions [J. M. Bowman, B. Gazdy, J. A. Bentley, T. J. Lee, and C. E. Dateo, J. Chem. Phys. 99, 308 (1993)] and novel full valence complete active space energies for CHN geometries. The various topographical features of the novel global potential energy surface are examined, and vibrational calculations are presented to characterize the minima associated to the HCN and CNH isomers. The quality of the new fit is compared with that of the seminumerical representation of Bowman et al. in terms of root-mean-squared deviations for stratified energy regimes.