Calculation of vibrational HDO energy levels: Analysis of perturbation theory series

Series of the Rayleigh-Schrödinger perturbation theory are analyzed and summated by the example of the HD16O molecule for vibrational energy levels. Particular attention is given to determining the location of singularities-branching points corresponding to the intersection of levels in the complex plane. Numerical analysis demonstrates the divergence of the series for states involved in the Fermi resonance; however, summation by the method of Padé-Hermite approximants makes it possible to reconstruct the levels by coefficients of the series with sufficient accuracy. It is found that resonance-coupled states have common branching points, which leads to the coincidence of series’ coefficients in high orders. Branching points’ characteristics permitting one to obtain a comparatively simple representation of high order corrections are determined.