A Prediction of the Vibration-Rotation Absorption Spectra of the \({\text{D}}_{2}^{{16}}{\text{O}}\) Molecule ν2 Band by Solving the Operator Perturbation Theory Direct Problem and a Refinement of the Effective Rotational Hamiltonians

Based on the solution of the direct vibrational-rotational problem with the Watson Hamiltonian using Van Vleck perturbation theory, employing the quantum-chemical (CCSD(T)/aug-cc-pVQZ) molecular geometry of molecule \({\text{D}}_{2}^{{16}}{\text{O}}\), sextic force field, and cubic dipole moment surface, absorption spectra and spectroscopic constants of effective rotational Hamiltonians (A-reduction) for the ground state and ν₂ band are predicted. Theoretical sixth-order vibrational (fourth for dipole operators) and third-order rotational perturbative methods are based on a systematic procedure for calculating rotational commutators using the normal ordering of cylindrical angular momentum operators. The obtained reduced spectroscopic constants are refined and augmented by being fitted into sets of experimental energy levels. The new effective Hamiltonians significantly improve the reproduction of the experimental data for J ≤ 30, and the computed intensities accurately reproduce the observed values.