A single-valued six-dimensional (6D) potential energy surface is determined for HeHCN by using the energy switching (ES) method, which utilizes a global double many-body expansion (DMBE) as well as a Legendre polynomial expansion for different energy regimes. The ES potential has a linear van der Waals well of 29.5 cm -1 which is about 4 cm -1 lower than that reported by Drucker et al. [J. Phys. Chem. 1995, 99, 2646 but agrees with one obtained by Atkins and Hutson [ J. Chem. Phys. 1996, 105, 440]. The effect of stretch of CN as well as CH bonds has also been investigated. With stretch, both the DMBE and ES schemes yield slightly deeper wells and steeper repulsion walls in the high energy region. Microwave transitions calculated at 15.9 and 31.3 GHz agree well with experiment and may be assigned to J ) 1 r 0 and J ) 2 r 1, respectively. This frequency is found to decrease with stretch. Finally, this ES potential energy surface has been compared with that of ArHCN reported by one of us [Chem. Phys. Lett. 1998, 297, 458]. Here, the question of transferability of the parameter which scales the Hatree-Fock energy in the rare gas-X (X ) H, C, and N) interaction from ArHCN to HeHCN has also been investigated. The transferability seems to work as it nicely brings out the position of the potential well and the rovibrational levels. The microwave transition, from the ground state (j ) 0, L ) 0, J ) 0) to state (1, 0, 1), found at 98.61 GHz in HeHCN can be compared with one found at about 162.48 GHz in ArHCN, in close agreement with experiment.

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