ИСТИНА

For a polyatomic molecule, second-order canonical perturbation theory (CPT2)1 expresses spectroscopic constants such as ωe and ωexe in the form of analytical expressions using parameters of the vibrational Hamiltonian and readily supports the calculation of energy levels for fundamentals, overtones and combination bands. The problem remains that the treatment of resonances is complicated and analytic expressions for spectroscopic constants for higher orders of perturbation theory are almost impossible to deduce. A powerful technique of using ladder operators significantly simplifies the procedure of canonical transformations for high orders of perturbation theory2. Combination of this technique with the one of normal ordering of operators allows developing an efficient computational algorithm for solving the anharmonic problem for a polyatomic molecule3,4. Within this method, the ladder operator representation of the transformed Hamiltonian is preserved, while numerical coefficients are kept as real numbers. In our Fortran computer implementation of the general scheme suggested by Sibert3,4, we lifted some limitations of the original program. Evaluation of vibrational transition probabilities in IR and Raman spectra could be efficiently accomplished using transformation S-functions obtained during canonical transformations. In the traditional form of CPT2 calculations using analytical formulas, the resonances are treated using a simple variational procedure, for which matrix elements of first (Fermi-type) and second-order (Darling-Dennison type) resonances are required. Analytical expressions for the latter ones are difficult to evaluate free of errors5. In the framework of the method employed by us, it is straightforward to introduce the uniform procedure of both detecting resonances, taking them into account in the final variational procedure, and presenting the final perturbative Hamiltonian in the form of a sum of the main diagonal Dunham-type expansion in powers of (vr+½) plus resonance-type expansions of the special general form. It is shown that for molecules studied the coefficients of perturbative Dunham-type Hamiltonians at 4th and 6th orders of CPT conform to the same rules as for diatomic molecules. The method is illustrated by results of vibrational analysis of several molecules. 1 H.H. Nielsen, Encyclopedia of Physics, Ed. S.Flügge, XXXVII/1 (1959) 173-313 2 Y.S.Makushkin, V.G. Tyuterev, "Methods of Perturbations and Effective Hamiltonians in Molecular Spectroscopy", Novosibirsk (1984) 3 E.L. Sibert III, J. Chem. Phys., 88 (1988) 4378-4390 4 E.L. Sibert III, Comput. Chem. Commun., 51 (1988) 149-160 5 D.A. Matthews, J. Vázquez, J.F. Stanton, Mol. Phys., 105 (2007) 2659-2666