||Molecular vibrations of the molecule HCN/CNH are examined using a combination of a minimum energy path (MEP) Hamiltonian and high order canonical perturbation theory (CPT), as suggested in a recent work [D. Sugny and M. Joyeux, J. Chem. Phys. 112, 31 (2000)]. In addition, the quantum analog of the classical CPT is presented and results obtained therefrom are compared to the classical ones. The MEP Hamiltonian is shown to provide an accurate representation of the original potential energy surface and a convenient starting point for the CPT. The CPT results are subsequently used to elucidate the molecular dynamics: It appears that the isomerization dynamics of HCN/CNH is very trivial, because the three vibrational modes remain largely decoupled up to and above the isomerization threshold. Therefore, the study of the three-dimensional HCN/CNH system can be split into the study of several one-dimensional bending subsystems, one for each value of the numbers upsilon (1) and upsilon (3) of quanta in the CH and CN stretches. In particular, application of high order CPT to the most precise available ab initio surface provides simple expressions (quadratic polynomials) for the calculation of the heights of the isomerization barrier and of the CNH minimum above the HCN minimum for each value of upsilon (1) and upsilon (3). (C) 2000 American Institute of Physics. [S0021-9606(00)00441-4].