Investigation of the vibrational dynamics of the HCN/CNH isomers through high order canonical perturbation theory

Molecular vibrations of the molecule HCN/CNH are examined using a combinati on of a minimum energy path (MEP) Hamiltonian and high order canonical pert urbation theory (CPT), as suggested in a recent work [D. Sugny and M. Joyeu x, J. Chem. Phys. 112, 31 (2000)]. In addition, the quantum analog of the c lassical CPT is presented and results obtained therefrom are compared to th e classical ones. The MEP Hamiltonian is shown to provide an accurate repre sentation of the original potential energy surface and a convenient startin g 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 decoupl ed 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 on e-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, a pplication of high order CPT to the most precise available ab initio surfac e provides simple expressions (quadratic polynomials) for the calculation o f 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 Americ an Institute of Physics. [S0021-9606(00)00441-4].