Semiclassical vibrational energies and transition frequencies for a Hamiltonian with stretch–bend potential energy coupling: Application of Fourier methods

Semiclassical methods are applied to determine transition frequencies and vibrational energies for the two-mode HC2 stretch–bend Hamiltonian recently studied by Swamy and Hase [J. Chem. Phys. 84, 361 (1986)]. The mean action spectral approximation for calculating transition frequencies is found to give results in excellent agreement with quantum variational values. The FFT EBK method is used to calculate vibrational eigenvalues for both nonresonant and 3:1 resonant states. Both approaches give results more accurate than those reported by Swamy and Hase using the DeLeon–Heller–Miller method. The presence of stretch–bend potential coupling is found to reduce the extent of classical chaos.