An adjusted global potential surface for HCN based on rigorous vibrational calculations

We report extensive trial and error modifications of the Murrell-Carter-Halonen potential surface for HCN to improve agreement with experiments on highly excited stretching and bending states. The vibrational calculations make use of an exact Hamiltonian for nonrotating HCN and use an exact formalism to obtain energies. Two experimental data bases are used to compare against the calculations. One is for highly excited stretch states, but with no bend excitation, and the other is for highly excited bend and CN stretch states, but with no CH stretch excitation. The combined data base consists of 58 vibrational energies for nonrotating HCN. The modifications applied are angular and stretch coordinate scaling and an angular-dependent potential scaling. In addition, the saddle point position is adjusted to agree with the results of a recent ab initio calculation.