ИСТИНА

A non-empirical prediction of high resolution molecular spectra and effective models provides a crucial supporting information for both deciphering of sophisticated experimental spectra and complementing empirical data missing in spectroscopic databases. While fully ab-initio calculation of vibration-rotation spectra involves solving Schrödinger equation with global rovibrational Hamiltonian, a conceptually different inverse problem utilizes effective models with restricted sets of parameters. The bridge between both problems can be built using the operator Van Vleck perturbation theory CVPT(n)that is able to calculate linelists, spectroscopic onstants and parameters of effective dipole moment operators. The major com- putational complication of CVPT(n) arises due to the necessity of evaluation of rotational commutators involving ladder operators of angular momentum (Jz , J± = Jx ∓ iJy). This problem can be efficiently solved using the normal ordering of these operators [1]. The rotational reduction for calculation of spectroscopic constants can also be efficiently accomplished with the aid of such normal ordering. Similarly, unitary transformations of the dipole moment operator can be made using the normal ordering with the Wigner D10,ε-functions (ε = −1, 0, +1). The obtained effective Hamiltonian and dipole moment operator can be further used for modeling of IR spectra and for iterative solution of inverse spectroscopic problem. A number of numerical examples for 3,4 and 5-atomic molecules is given, demonstrating an efficiency of the developed approach.[2] 1 Chang, X., Millionshchikov, D.V., Efremov, I.M. et al. 2023 J. Chem. Phys. 158, 104802 2 Krasnoshchekov, S.V., Efremov, I.M., Polyakov, I.V. and Millionshchikov, D.V. 2024 J. Chem. Phys., 161, 234105