ИСТИНА

Among the major approaches for modelling of vibration-rotation energy / IR intensity line lists, the direct problem is often solved by the variational method (VM) while the inverse problem is typically solved by adjusting sets of spectroscopic constants participating in analytic effective Hamiltonians (EH). The VCI method is very efficient for small molecules but requires a subtle empirical correction of ab initio data. The EF method is efficient as well, but the choice of adjustable parameters is somewhat equivocal and typically suffers by strong correlations. A meaningful alternative to VM or EF approaches is the solution of the vibration-rotation problem with Watson Hamiltonian by means of applying the operator version of the canonical Van Vleck perturbation theory (CVPT) in the second, fourth or even higher orders.1-3 Although the straightforward implementation of CVPT depends on the quality of ab initio potential energy and dipole moment surfaces and is less reliable than VM for strongly excited states and large amplitude motions, it has much better scaling properties and produces unambiguous sets of spectroscopic constants than can be further adjusted via EH method. This is very convenient as parameters with linear dependencies within EF model can be fixed. Moreover, such ab initio estimation of adjustable EF parameters can be served as a reliable guide. In this work, we employed a practical and efficient theoretical approach based on normal ordering of arbitrary products of rotational operators with reducing them to a sum of products Jza J+b J–c. For evaluation of vibration-rotation IR intensities a similar procedure is applied for obtaining normally ordered similar products of both direct cosine and rotational operators. As the result, all necessary commutators within CVPT and subsequent evaluation of unitary transformed dipole moment operators can be evaluated. This theoretical scheme is implemented in fast Fortran 95 code ANCO in numerical-analytic form. For treating larger molecules this code can be parallelized. The efficiency of ANCO code is demonstrated by juxtaposition of modelled spectra of SO2 molecule with HITRAN data and by comparing spectroscopic constants with literature data. References [1] V. G. Tyuterev, S. A. Tashkun, H. Seghir, SPIE Proceedings Series 5311, 164, 2004 [2] J. Lamouroux, S. A. Tashkun, V. G. Tyuterev, Chem. Phys. Lett. 452, 225, 2008 [3] V. Tyuterev, S. Tashkun, M. Rey, R. Kochanov, A. Nikitin, T. Delahaye J. Phys. Chem. A 117, 13779, 2013