ИСТИНА

Perturbation theory has been one from the main approaches for understanding rovibrational spectroscopy and serves as an alternative to the variational method [1]. With the development of the quantum chemistry the second order vibrational perturbation theory (VPT2) has become a standard method for interpreting experimental high-resolution spectra [2]. However, gaining a comprehensive understanding of the spectroscopic characteristics expressed through the perturbation series in a wide spectral region requires an implementation of the high-order perturbation approach, which have to be established not only on a deep understanding of the theory, but also a high-performance program algorithm. In this work, we employ the high-order contact transformation approach, followed by the Rotational Watson A-reduction up to the 8-th order [3] and based on a straightforward normal ordering of angular momentum operators in the form Jz^a J+^b J–^c [4]. For evaluation of vibration-rotation transition intensities a similar procedure is applied for angular momentum operators and Winger D function in order to compute the effective dipole moment operator. The theoretical scheme was implemented in the ANCO program written in Fortran95 using a numerical-analytic algorithm. To demonstrate the effectiveness of the implemented approach, we compare the spectroscopic parameters (including the octic parameters L) with the theoretical [3,5] and fitting values [6,7] from the literature, as well as the modeled rovibrational transitions of SO2 molecule with HITRAN database up to the first triad. References: 1. I. S. Makushkin, V. G. Tyuterev, Perturbation methods and effective Hamiltonians in molecular spectroscopy, Novosibirsk Izdatelstvo Nauka (1984). 2. C. Puzzarini, J. Bloino, N. Tasinato, V. Barone, Chem. Rev. 119, 8131-8191 (2019). 3. J. K. Watson, J. Mol. Struct. 795, 263–270 (2006). 4. X. Chang, D.V. Millionshchikov, I.M. Efremov, S.V. Krasnoshchekov, J. Chem. Phys. 158, 104802 (2023) 5. J. M. L. Martin, J. Chem. Phys. 108, 2791–2800 (1998). 6. O. Ulenikov, G. Onopenko, O. Gromova, E. Bekhtereva, V.-M. Horneman, J. Quant. Spectrosc. Radiat. Transfer 130, 220–232 (2013). 7. O. Ulenikov, E. Bekhtereva, O. Gromova, M. Quack, G. Mellau, C. Sydow, S. Bauerecker, J. Quant. Spectrosc. Radiat. Transfer 210, 141–155 (2018).