ИСТИНА

The large order Rayleigh-Schrödinger perturbation theory (RSPT) and the quartic Pade-Hermite approximant (PHA) was used for studying the resonance structure of the linear molecule acetylene, combined with an ab initio quartic force field at the level CCSD(T).1,2 Large order perturbation series of certain vibrational states, calculated by the advanced recursive RSPT formula for a degenerate case, demonstrate strong divergence behaviours and regular anti-sign patterns, indicating the existence of strong couplings between corresponding states in the molecule (fig.1). Such patterns are well controlled by the singularities of the eigenvalues on the complex plane through the Darboux and the Katz theorems, and are further confirmed from the singular points analysis followed by a resummation procedure using a quartic Pade-Hermite approximant. As the result, all classical resonances, including the stretching Darling-Dennison coupling, three stretch-bend couplings and the bending Darling-Dennison coupling, were confirmed by the applied method. Two interpolyad resonances, proposed by Herman et.al.,3 were supported by our calculation. In addition, we proposed an independent new interpolyad resonance K2/4444. The complete analysis of the RSPT series and their singularities provides a comprehensive resonance structure of the acetylene. Fig. 1: The selected RSPT series of vibrational states: (a) v2+v4+v5 and v3; (b) v1 and v2+2v5. References [1] Xuanhao Chang, Egor O. Dobrolyubov, Sergey V. Krasnoshchekov, Phys. Chem. Chem. Phys. 24, 6655. [2] J.M.L. Martin, T.J. Lee, P.R. Taylor, J. Chem. Phys. 108, 676, 1998 [3] B. Amyay, M. Herman, A. Fayt, A. Campargue, S. Kassi, J. Mol. Spectrosc. 267, 80, 2011