Application of quasi-degenerate perturbation theory to the calculation of rotational energy levels of methane vibrational polyads

摘要: In previous works, we have introduced an alternative perturbation scheme to find approximate solutions of the spectral problem for rotation-vibration molecular Hamiltonian. An important feature our approach is that zero order Hamiltonian direct product a purely vibrational with identity on rotational degrees freedom. The convergence method methane ground state was very satisfactory and predictions were quantitative. present article, provide further details implementation in degenerate quasi-degenerate cases. version tested excited polyads methane, results are assessed respect variational treatment. optimal choice size spaces determined by trade-off between speed series computational effort obtain effective super-Hamiltonian.